cmd/gc: avoid %#x of 0
cmd/gc: avoid %#x of 0

Plan 9 and Go's lib9/fmt disagree on whether %#x includes the 0x prefix
when printing 0, because ANSI C gave bad advice long ago.

Avoiding that case makes binaries compiled on different systems compatible.

R=ken2
CC=akumar, golang-dev
http://codereview.appspot.com/6814066
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go