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.

Change-Id: I2eef8d550bbecba3a087869df2c7351280c77290
Reviewed-on: https://go-review.googlesource.com/87136
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/go/internal/work/exec.go

@@ -1849,9 +1849,11 @@ 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
 }