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
https://golang.org/cl/5094042

test/fixedbugs/bug337.go

@@ -14,6 +14,6 @@
 package main
 
 func main() {
-	len("foo")	// ERROR "len"
+	len("foo")	// ERROR "len|value computed is not used"
 }