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

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang