New gccgo error message; match both compilers with one string.

8g:
runtime.go:19: cannot refer to unexported name runtime.printbool

gccgo:
runtime.go:19:10: error: invalid reference to unexported identifier ‘runtime.printbool’

R=rsc
CC=golang-dev
https://golang.org/cl/194157

test/runtime.go

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