[dev.typeparams] all: merge master (4711bf30) into dev.typeparams

Conflicts:

- src/cmd/compile/internal/walk/builtin.go

  On dev.typeparams, CL 330194 changed OCHECKNIL to not require manual
  SetTypecheck(1) anymore; while on master, CL 331070 got rid of the
  OCHECKNIL altogether by moving the check into the runtime support
  functions.

- src/internal/buildcfg/exp.go

  On master, CL 331109 refactored the logic for parsing the
  GOEXPERIMENT string, so that it could be more easily reused by
  cmd/go; while on dev.typeparams, several CLs tweaked the regabi
  experiment defaults.

Merge List:

+ 2021-06-30 4711bf30 doc/go1.17: linkify "language changes" in the runtime section
+ 2021-06-30 ed56ea73 path/filepath: deflake TestEvalSymlinksAboveRoot on darwin
+ 2021-06-30 c080d032 cmd/dist: pass -Wno-unknown-warning-option in swig_callback_lto
+ 2021-06-30 7d0e9e6e image/gif: fix typo in the comment (io.ReadByte -> io.ByteReader)
+ 2021-06-30 0fa3265f os: change example to avoid deprecated function
+ 2021-06-30 d19a5333 image: add Uniform.RGBA64At and Rectangle.RGBA64At
+ 2021-06-30 c45e800e crypto/x509: don't fail on optional auth key id fields
+ 2021-06-29 f9d50953 net: fix failure of TestCVE202133195
+ 2021-06-29 e294b8a4 doc/go1.17: fix typo "MacOS" -> "macOS"
+ 2021-06-29 3463852b math/big: fix typo of comment (`BytesScanner` to `ByteScanner`)
+ 2021-06-29 fd4b587d cmd/compile: suppress details error for invalid variadic argument type
+ 2021-06-29 e2e05af6 cmd/internal/obj/arm64: fix an encoding error of CMPW instruction
+ 2021-06-28 4bb0847b cmd/compile,runtime: change unsafe.Slice((*T)(nil), 0) to return []T(nil)
+ 2021-06-28 1519271a spec: change unsafe.Slice((*T)(nil), 0) to return []T(nil)
+ 2021-06-28 5385e238 runtime/internal/atomic: drop Cas64 pointer indirection in comments
+ 2021-06-28 956c81bf cmd/go: add GOEXPERIMENT to `go env` output
+ 2021-06-28 a1d27269 cmd/go: prep for 'go env' refactoring
+ 2021-06-28 901510ed cmd/link/internal/ld: skip the windows ASLR test when CGO_ENABLED=0
+ 2021-06-28 361159c0 cmd/cgo: fix 'see gmp.go' to 'see doc.go'
+ 2021-06-27 c95464f0 internal/buildcfg: refactor GOEXPERIMENT parsing code somewhat
+ 2021-06-25 ed01ceaf runtime/race: use race build tag on syso_test.go
+ 2021-06-25 d1916e5e go/types: in TestCheck/issues.src, import regexp/syntax instead of cmd/compile/internal/syntax
+ 2021-06-25 5160896c go/types: in TestStdlib, import from source instead of export data
+ 2021-06-25 d01bc571 runtime: make ncgocall a global counter

Change-Id: I1ce4a3b3ff7c824c67ad66dd27d9d5f1d25c0023