Match gccgo error messages.

import1.go:12:8: error: redefinition of ‘bufio’
import1.go:11:8: note: previous definition of ‘bufio’ was here
import1.go:16:2: error: redefinition of ‘fmt’
import1.go:15:2: note: previous definition of ‘fmt’ was here
import1.go:11:8: error: imported and not used: bufio

R=rsc
CC=golang-dev
https://golang.org/cl/194165

test/import1.go

@@ -8,10 +8,10 @@
 
 package main
 
-import "bufio"	// GCCGO_ERROR "previous"
+import "bufio"	// GCCGO_ERROR "previous|not used"
 import bufio "os"	// ERROR "redeclared|redefinition|incompatible"
 
 import (
-	"fmt";	// GCCGO_ERROR "previous"
+	"fmt";	// GCCGO_ERROR "previous|not used"
 	fmt "math";	// ERROR "redeclared|redefinition|incompatible"
 )