gopack: allow ELF/Mach-O objects in .a files without clearing allobj
gopack: allow ELF/Mach-O objects in .a files without clearing allobj

R=iant
CC=golang-dev
http://codereview.appspot.com/3503041
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go