cmd/go: check for another GCC error message

GCC always recognizes the -fsplit-stack option, but then tests whether
    it is supported by the selected target. If not, it reports
        cc1: error: ‘-fsplit-stack’ is not supported by this compiler configuration
    Check for that error message when deciding whether a compiler option works.
    
    Reviewed-on: https://go-review.googlesource.com/87137

From-SVN: r256433

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-c22eb29a62b4fd72ad2ea09ebe5fcea5b8ed78b8
+87df767807acac466edb3bb6445ad83a48141d17
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/cmd/go/internal/work/exec.go

@@ -1857,9 +1857,11 @@ func (b *Builder) gccSupportsFlag(compiler []string, flag string) bool {
 	// GCC says "unrecognized command line option".
 	// clang says "unknown argument".
 	// Older versions of GCC say "unrecognised debug output level".
+	// For -fsplit-stack GCC says "'-fsplit-stack' is not supported".
 	supported := !bytes.Contains(out, []byte("unrecognized")) &&
 		!bytes.Contains(out, []byte("unknown")) &&
-		!bytes.Contains(out, []byte("unrecognised"))
+		!bytes.Contains(out, []byte("unrecognised")) &&
+		!bytes.Contains(out, []byte("is not supported"))
 	b.flagCache[key] = supported
 	return supported
 }