bytes: annotate assembly functions with //go:noescape

R=golang-dev, agl, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/7299064

src/pkg/bytes/bytes.go

@@ -37,10 +37,6 @@ func Compare(a, b []byte) int {
 	return 0
 }
 
-// Equal returns a boolean reporting whether a == b.
-// A nil argument is equivalent to an empty slice.
-func Equal(a, b []byte) bool
-
 func equalPortable(a, b []byte) bool {
 	if len(a) != len(b) {
 		return false