runtime/cgo: fix stackguard on FreeBSD/amd64
runtime/cgo: fix stackguard on FreeBSD/amd64

A cursory reading of the cgo code suggests this
should be necessary, though I don't have access
to a FreeBSD machine for testing.

R=rsc, adg
CC=golang-dev
http://codereview.appspot.com/3746047

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

wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old