arm: fix build2, tweak test/nul1.go arm exclusion
arm: fix build2, tweak test/nul1.go arm exclusion

R=rsc
CC=golang-dev
http://codereview.appspot.com/1866044
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go