Recognize gccgo error message.

runtime.go:19:10: error: reference to undefined identifier ‘runtime.printbool’

R=rsc
http://go/go-review/1018018

test/runtime.go

@@ -16,5 +16,5 @@ package main
 import "runtime"
 
 func main() {
-	runtime.printbool(true);	// ERROR "cannot refer"
+	runtime.printbool(true);	// ERROR "cannot refer|undefined identifier"
 }