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

tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go