runtime/cgo: fix build on freebsd/arm
runtime/cgo: fix build on freebsd/arm

This CL is in preparation to make cgo work on freebsd/arm.

LGTM=iant
R=iant
CC=golang-codereviews
https://codereview.appspot.com/60500044

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