gc: fix mkbuiltin for Plan 9 build
gc: fix mkbuiltin for Plan 9 build

. Replaced Posix #includes with u.h and libc.h.
. Replaced fprintf(stderr,...); exit(1); with sysfatal() calls.

R=golang-dev
CC=golang-dev, rsc
http://codereview.appspot.com/4823047

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