unix: omit success message if generation of ptrace definition fails

If generating ptrace definitions for a particular pair fails for any
reason, a success message would still be shown. Avoid it in case there
was a failure.

Change-Id: Ica59c888586c055e8ef333a0db00bde2c4d15145
Reviewed-on: https://go-review.googlesource.com/73874
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/linux/mkall.go

@@ -178,6 +178,7 @@ func main() {
 	for _, p := range ptracePairs {
 		if err := generatePtracePair(p.a1, p.a2); err != nil {
 			fmt.Printf("%v\n***** FAILURE: %s/%s *****\n\n", err, p.a1, p.a2)
+			ok = false
 		}
 	}
 	if ok {