[dev.regabi] all: merge master (dab3e5af) into dev.regabi

This merge had two conflicts to resolve:

1. The embed code on master had somewhat substantially diverged, so
this CL tediously backported the changes to dev.regabi. In particular,
I went through all of the embed changes to gc/{embed,noder,syntax}.go
and made sure the analogous code on dev.regabi in noder/noder.go and
staticdata/embed.go mirrors it.

2. The init-cycle reporting code on master was extended slightly to
track already visited declarations to avoid exponential behavior. The
same fix is applied on dev.regabi, just using ir.NameSet instead of
map[ir.Node]bool.

Conflicts:

- src/cmd/compile/internal/gc/embed.go
- src/cmd/compile/internal/gc/noder.go
- src/cmd/compile/internal/gc/syntax.go
- src/cmd/compile/internal/pkginit/initorder.go
- src/embed/internal/embedtest/embed_test.go
- src/go/types/stdlib_test.go

Merge List:

+ 2021-01-22 dab3e5af runtime: switch runtime to libc for openbsd/amd64
+ 2021-01-22 a1b53d85 cmd/go: add documentation for test and xtest fields output by go list
+ 2021-01-22 b268b607 runtime: remove pthread_kill/pthread_self for openbsd
+ 2021-01-22 ec405176 runtime: fix typo in mgcscavenge.go
+ 2021-01-22 7ece3a7b net/http: fix flaky TestDisableKeepAliveUpgrade
+ 2021-01-22 50cba050 time: clarify Timer.Reset behavior on AfterFunc Timers
+ 2021-01-22 cf10e69f doc/go1.16: mention net/http.Transport.GetProxyConnectHeader
+ 2021-01-22 ec1b9452 doc/go1.16: mention path/filepath.WalkDir
+ 2021-01-22 11def3d4 doc/go1.16: mention syscall.AllThreadsSyscall
+ 2021-01-21 07b02356 doc/go1.16: add notes about package-specific fs.FS changes
+ 2021-01-21 e2b4f1fe doc/go1.16: minor formatting fix
+ 2021-01-21 9f43a9e0 doc/go1.16: mention new debug/elf constants
+ 2021-01-21 3c2f11ba cmd/go: overwrite program name with full path
+ 2021-01-21 953d1fec all: introduce and use internal/execabs
+ 2021-01-21 b186e4d7 cmd/go: add test case for cgo CC setting
+ 2021-01-21 5a8a2265 cmd/cgo: report exec errors a bit more clearly
+ 2021-01-21 46e2e2e9 cmd/go: pass resolved CC, GCCGO to cgo
+ 2021-01-21 3d40895e runtime: switch openbsd/arm64 to pthreads
+ 2021-01-21 d95ca913 crypto/elliptic: fix P-224 field reduction
+ 2021-01-20 ecf4ebf1 cmd/internal/moddeps: check content of all modules in GOROOT
+ 2021-01-20 d2d155d1 runtime: don't adjust timer pp field in timerWaiting status
+ 2021-01-20 803d18fc cmd/go: set Incomplete field on go list output if no files match embed
+ 2021-01-20 6e243ce7 cmd/go: have go mod vendor copy embedded files in subdirs
+ 2021-01-20 be28e5ab cmd/go: fix mod_get_fallback test
+ 2021-01-20 928bda4f runtime: convert openbsd/amd64 locking to libc
+ 2021-01-19 824f2d63 cmd/go: allow go fmt to complete when embedded file is missing
+ 2021-01-19 0575e35e cmd/compile: require 'go 1.16' go.mod line for //go:embed
+ 2021-01-19 ccb2e906 cmd/link: exit before Asmb2 if error
+ 2021-01-19 ca5774a5 embed: treat uninitialized FS as empty
+ 2021-01-19 d047c91a cmd/link,runtime: switch openbsd/amd64 to pthreads
+ 2021-01-19 61debffd runtime: factor out usesLibcall
+ 2021-01-19 9fed39d2 runtime: factor out mStackIsSystemAllocated
+ 2021-01-18 dbab0798 runtime: free Windows event handles after last lock is dropped
+ 2021-01-18 5a8fbb0d os: do not close syscall.Stdin in TestReadStdin
+ 2021-01-15 682a1d21 runtime: detect errors in DuplicateHandle
+ 2021-01-15 9f83418b cmd/link: remove GOROOT write in TestBuildForTvOS
+ 2021-01-15 ec947016 cmd/compile: allow embed into any string or byte slice type
+ 2021-01-15 54198b04 cmd/compile: disallow embed of var inside func
+ 2021-01-15 b386c735 cmd/go: fix go generate docs
+ 2021-01-15 bb5075a5 syscall: remove RtlGenRandom and move it into internal/syscall
+ 2021-01-15 1deae0b5 os: invoke processKiller synchronously in testKillProcess
+ 2021-01-15 ff196c3e crypto/x509: update iOS bundled roots to version 55188.40.9
+ 2021-01-14 e125ccd1 cmd/go: in 'go mod edit', validate versions given to -retract and -exclude
+ 2021-01-14 eb330020 cmd/dist, cmd/go: pass -arch for C compilation on Darwin
+ 2021-01-14 84e8a06f cmd/cgo: remove unnecessary space in cgo export header
+ 2021-01-14 0c86b999 cmd/test2json: document passing -test.paniconexit0
+ 2021-01-14 91357958 cmd/go/internal/load: report positions for embed errors
+ 2021-01-14 d9b79e53 cmd/compile: fix wrong complement for arm64 floating-point comparisons
+ 2021-01-14 c73232d0 cmd/go/internal/load: refactor setErrorPos to PackageError.setPos
+ 2021-01-14 6aa28d3e go/build: report positions for go:embed directives
+ 2021-01-13 7eb31d99 cmd/go: add hints to more missing sum error messages
+ 2021-01-12 ba76567b cmd/go/internal/modload: delete unused *mvsReqs.next method
+ 2021-01-12 665def2c encoding/asn1: document unmarshaling behavior for IMPLICIT string fields
+ 2021-01-11 81ea89ad cmd/go: fix non-script staleness checks interacting badly with GOFLAGS
+ 2021-01-11 75930902 doc: update editors.html for Go 1.16
+ 2021-01-11 c3b4c709 cmd/internal/objfile: don't require runtime.symtab symbol for XCOFF
+ 2021-01-08 59bfc18e cmd/go: add hint to read 'go help vcs' to GOVCS errors
+ 2021-01-08 cd6f3a54 cmd/go: revise 'go help' documentation for modules
+ 2021-01-08 6192b987 cmd/go: make hints in error messages more consistent
+ 2021-01-08 25886cf4 cmd/go: preserve sums for indirect deps fetched by 'go mod download'
+ 2021-01-08 62508339 runtime/metrics: mark histogram metrics as cumulative
+ 2021-01-08 8f6a9acb runtime/metrics: remove unused StopTheWorld Description field
+ 2021-01-08 6598c656 cmd/compile: fix exponential-time init-cycle reporting
+ 2021-01-08 fefad1dc test: fix timeout code for invoking compiler
+ 2021-01-08 6728118e cmd/go: pass signals forward during "go tool"
+ 2021-01-08 e65c543f go/build/constraint: add parser for build tag constraint expressions
+ 2021-01-08 0c5afc4f testing/fstest,os: clarify racy behavior of TestFS
+ 2021-01-08 32afcc94 runtime/metrics: change unit on *-by-size metrics to match bucket unit
+ 2021-01-08 c6513bca io/fs: minor corrections to Glob doc
+ 2021-01-08 304f769f cmd/compile: don't short-circuit copies whose source is volatile
+ 2021-01-08 ae977171 runtime,runtime/metrics: use explicit histogram boundaries
+ 2021-01-08 a9ccd2d7 go/build: skip string literal while findEmbed
+ 2021-01-08 d92f8add archive/tar: fix typo in comment
+ 2021-01-08 cab12021 cmd/link: accept extra blocks in TestFallocate
+ 2021-01-08 ee4d3224 io/fs: minor corrections to Glob release date
+ 2021-01-08 54bd1ccc cmd: update to latest golang.org/x/tools
+ 2021-01-07 9ec21a8f Revert "reflect: support multiple keys in struct tags"
+ 2021-01-07 091414b5 io/fs: correct WalkDirFunc documentation
+ 2021-01-07 9b55088d doc/go1.16: add release note for disallowing non-ASCII import paths
+ 2021-01-07 fa90aaca cmd/compile: fix late expand_calls leaf type for OpStructSelect/OpArraySelect
+ 2021-01-07 7cee66d4 cmd/go: add documentation for Embed fields in go list output
+ 2021-01-07 e60cffa4 html/template: attach functions to namespace
+ 2021-01-07 6da2d3b7 cmd/link: fix typo in asm.go
+ 2021-01-07 df81a158 runtime: check mips64 VDSO clock_gettime return code
+ 2021-01-06 4787e906 crypto/x509: rollback new CertificateRequest fields
+ 2021-01-06 c9658bee cmd/go: make module suggestion more friendly
+ 2021-01-06 4c668b25 runtime/metrics: fix panic message for Float64Histogram
+ 2021-01-06 d2131704 net/http/httputil: fix deadlock in DumpRequestOut
+ 2021-01-05 3e1e13ce cmd/go: set cfg.BuildMod to "readonly" by default with no module root
+ 2021-01-05 0b0d0049 cmd/go: pass embedcfg to gccgo if supported
+ 2021-01-05 1b85e7c0 cmd/go: don't scan gccgo standard library packages for imports
+ 2021-01-05 6b37b15d runtime: don't take allglock in tracebackothers
+ 2021-01-04 9eef49cf math/rand: fix typo in comment
+ 2021-01-04 b01fb2af testing/fstest: fix typo in error message
+ 2021-01-01 3dd58676 doc: 2021 is the Year of the Gopher
+ 2020-12-31 95ce805d io/fs: remove darwin/arm64 special condition
+ 2020-12-30 20d0991b lib/time, time/tzdata: update tzdata to 2020f
+ 2020-12-30 ed301733 misc/cgo/testcarchive: remove special flags for Darwin/ARM
+ 2020-12-30 0ae2e032 misc/cgo/test: enable TestCrossPackageTests on darwin/arm64
+ 2020-12-29 780b4de1 misc/ios: fix wording for command line instructions
+ 2020-12-29 b4a71c95 doc/go1.16: reference misc/ios/README for how to build iOS programs
+ 2020-12-29 f83e0f66 misc/ios: add to README how to build ios executables
+ 2020-12-28 4fd94558 io/fs: fix typo in comment

Change-Id: I2f257bbc5fbb05f15c2d959f8cfe0ce13b083538