libmach: fix build on arm host
libmach: fix build on arm host

Fixes issue 1079.

R=r, trufae, fgergo
CC=golang-dev
http://codereview.appspot.com/2103048
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go