test: match gccgo error message for bug337.go
test: match gccgo error message for bug337.go

bug337.go:17:2: error: value computed is not used

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5094042
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go