runtime: disable gc test on non-amd64 systems
runtime: disable gc test on non-amd64 systems

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6210062
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go