cgo: use packed struct to fix Windows behavior
cgo: use packed struct to fix Windows behavior

R=golang-dev, mattn.jp, jcowgar, iant
CC=golang-dev
http://codereview.appspot.com/4535080
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go