cmd/internal/gc: add -buildid flag to write build ID into object file

The build ID is an opaque token supplied by the build system.
The compiler writes it out early in the Go export metadata
(the second line), in a way that does not bother existing readers.

The intent is that the go command can use this to store information
about the sources for the generated code, so that it can detect
stale packages even in cases (like removed files) where mtimes fail.

Change-Id: Ib5082515d6cde8a07a8d4b5c69d1e8e4190cb5e1
Reviewed-on: https://go-review.googlesource.com/9153Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/export.go

@@ -361,6 +361,9 @@ func dumpsym(s *Sym) {
 func dumpexport() {
 	lno := lineno
 
+	if buildid != "" {
+		fmt.Fprintf(bout, "build id %q\n", buildid)
+	}
 	fmt.Fprintf(bout, "\n$$\npackage %s", localpkg.Name)
 	if safemode != 0 {
 		fmt.Fprintf(bout, " safe")

src/cmd/compile/internal/gc/lex.go

@@ -28,11 +28,12 @@ var yylast int
 
 var imported_unsafe int
 
-var goos string
-
-var goarch string
-
-var goroot string
+var (
+	goos    string
+	goarch  string
+	goroot  string
+	buildid string
+)
 
 var (
 	Debug_wb     int
@@ -203,6 +204,7 @@ func Main() {
 	obj.Flagfn0("V", "print compiler version", doversion)
 	obj.Flagcount("W", "debug parse tree after type checking", &Debug['W'])
 	obj.Flagstr("asmhdr", "write assembly header to `file`", &asmhdr)
+	obj.Flagstr("buildid", "record `id` as the build id in the export metadata", &buildid)
 	obj.Flagcount("complete", "compiling complete package (no C or assembly)", &pure_go)
 	obj.Flagstr("d", "print debug information about items in `list`", &debugstr)
 	obj.Flagcount("e", "no limit on number of errors reported", &Debug['e'])