cmd/go: fix gccSupportsNoPie for old GCC's that don't exit 0

GCC 4.8 exits 1 on an unrecognized option, but GCC 4.4 and 4.5 exit 0.
I didn't check other versions, or try to figure out just when this
changed.

Fixes #13937.

Change-Id: If193e9053fbb535999c9bde99f430f465a8c7c57
Reviewed-on: https://go-review.googlesource.com/18597
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/build.go

@@ -2941,8 +2941,8 @@ func (b *builder) gccSupportsNoPie() bool {
 	cmd := exec.Command(cmdArgs[0], cmdArgs[1:]...)
 	cmd.Dir = b.work
 	cmd.Env = envForDir(cmd.Dir, os.Environ())
-	err := cmd.Run()
-	return err == nil
+	out, err := cmd.CombinedOutput()
+	return err == nil && !bytes.Contains(out, []byte("unrecognized"))
 }
 
 // gccArchArgs returns arguments to pass to gcc based on the architecture.