cmd/ld: support zero-filled section for Mach-O files
cmd/ld: support zero-filled section for Mach-O files

R=golang-dev, r, dave, rsc
CC=golang-dev
http://codereview.appspot.com/6492069
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go