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>

ssrl/go,ssrl/go,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,dsriram/golang,ssrl/go,apparentlymart/golang,tv42/old-go,dsriram/golang,gnanderson/go,gnanderson/go,ssrl/go,gnanderson/go,ssrl/go,apparentlymart/golang,tv42/old-go,ssrl/go,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,ssrl/go,apparentlymart/golang,gnanderson/go,gnanderson/go,tv42/old-go,dsriram/golang,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,apparentlymart/golang