[dev.regabi] all: merge master (bf0f7c9d) into dev.regabi

This merge involved two merge conflicts:

1. walk's ascompatee code has been substantially refactored on
dev.regabi, so CL 285633 is ported to the new style.

2. The os.TestDirFS workaround added in CL 286213 can be removed now
that #42637 has been fixed by CL 285720.

Conflicts:

- src/cmd/compile/internal/gc/walk.go
- src/os/os_test.go

Merge List:

+ 2021-01-25 bf0f7c9d doc/go1.16: mention os.DirFS in os section
+ 2021-01-25 deaf29a8 cmd/compile: fix order-of-assignment issue w/ defers
+ 2021-01-25 ad2ca26a doc/go1.16: mention os.DirEntry and types moved from os to io/fs
+ 2021-01-25 a51921fa doc/go1.16: mention new testing/iotest functions
+ 2021-01-25 e6b6d107 doc/go1.16: mention deprecation of io/ioutil
+ 2021-01-25 96a27636 doc/go1.16: mention go/build changes
+ 2021-01-25 3d85c69a html/template: revert "avoid race when escaping updates template"
+ 2021-01-25 54514c6b cmd/go: fix TestScript/cgo_path, cgo_path_space when CC set
+ 2021-01-25 6de8443f doc/asm: add a section on go_asm.h, clean up go_tls.h section
+ 2021-01-25 54b251f5 lib/time, time/tzdata: update tzdata to 2021a
+ 2021-01-25 ff82cc97 os: force consistent mtime before running fstest on directory on Windows
+ 2021-01-25 044f937a doc/go1.16: fix WalkDir and Walk links
+ 2021-01-23 b634f5d9 doc/go1.16: add crypto/x509 memory optimization
+ 2021-01-23 9897655c doc/go1.16: reword ambiguously parsable sentence
+ 2021-01-23 cd99385f cmd/internal/obj/arm64: fix VMOVQ instruction encoding error
+ 2021-01-23 66ee8b15 runtime: restore cgo_import_dynamic for libc.so on openbsd
+ 2021-01-22 25c39e4f io/ioutil: fix example test for WriteFile to allow it to run in the playground
+ 2021-01-22 eb21b31e runtime: define dummy msanmove
+ 2021-01-22 3a778ff5 runtime: check for g0 stack last in signal handler
+ 2021-01-22 a2cef9b5 cmd/go: don't lookup the path for CC when invoking cgo

Change-Id: I651949f9eb18b57e3c996c4f3b2b3bf458bc5d97