cmd/gc: emit code for extern = <N>

https://codereview.appspot.com/152700045/ made it possible for struct literals assigned to globals to use <N> as the RHS. Normally, this is to zero out variables on first use. Because globals are already zero (or their linker initialized value), we just ignored this.

Now that <N> can occur from non-initialization code, we need to emit this code. We don't use <N> for initialization of globals any more, so this shouldn't cause any excessive zeroing.

Fixes issue 8961.

LGTM=rsc
R=golang-codereviews, rsc
CC=bradfitz, golang-codereviews
https://codereview.appspot.com/154540044