[release-branch.go1.15] all: merge master into release-branch.go1.15

ba9e1088 cmd: update golang.org/x/xerrors
027d7241 encoding/binary: read at most MaxVarintLen64 bytes in ReadUvarint
6f08e89e cmd/go: fix error stacks when there are scanner errors
f2352750 net/http: fix cancelation of requests with a readTrackingBody wrapper
f9233742 runtime/race: fix ppc64le build
e49b2308 runtime/race: rebuild some .syso files to remove getauxval dependency
10523c0e doc/go1.15: fix a few trivial inconsistencies
7388956b cmd/cgo: fix mangling of enum and union types
b56791cd runtime: validate candidate searchAddr in pageAlloc.find
10374e24 testing: fix quotation marks
7f860804 cmd/compile: don't addLocalInductiveFacts if there is no direct edge from if block to phi block
54e75e8f crypto/ed25519: remove s390x KDSA implementation
6b4dcf19 runtime: hold sched.lock over globrunqputbatch in runqputbatch
85afa2eb runtime: ensure startm new M is consistently visible to checkdead
c4fed255 cmd/compile: add floating point load+op operations to addressing modes pass
19a932ce cmd/link: don't mark shared library symbols reachable unconditionally
8696ae82 syscall: use correct file descriptor in dup2 fallback path
9591515f runtime, sync: add copyright headers to new files
074f2d80 doc/go1.15: surface the crypto/x509 CommonName deprecation note

Change-Id: I0bfcff1fc2de723960909d9dda718fee6abc2912