[dev.boringcrypto.go1.16] all: merge go1.16.15 into dev.boringcrypto.go1.16

src/cmd/compile/internal/ssa/gen/RISCV64.rules

src/cmd/compile/internal/ssa/rewriteRISCV64.go

src/cmd/go/internal/modfetch/coderepo.go

src/cmd/go/internal/modfetch/coderepo_test.go

src/net/dnsclient_unix.go

src/net/dnsclient_unix_test.go

src/regexp/syntax/parse.go

src/regexp/syntax/parse_test.go

src/runtime/export_test.go

src/runtime/histogram.go

src/runtime/histogram_test.go

test/fixedbugs/issue51101.go