ld: fix Mach-O X86_64_RELOC_SIGNED relocations
ld: fix Mach-O X86_64_RELOC_SIGNED relocations

Fixes issue 1658.

R=rsc
CC=golang-dev
http://codereview.appspot.com/4344066

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go