syscall: don't clobber source files if mksyscall.pl fails.
syscall: don't clobber source files if mksyscall.pl fails.

R=golang-dev, minux.ma, r, rsc
CC=golang-dev
http://codereview.appspot.com/6353069

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go