test: match gccgo import error messages

Gccgo doesn't have the same equivalent of file name and
package as the gc compiler, so the error messages are
necessarily different.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/40510048
diff --git a/test/import1.go b/test/import1.go
index d2bb55cb..2433b5f 100644
--- a/test/import1.go
+++ b/test/import1.go
@@ -15,5 +15,5 @@
 import (
 	"fmt"	// GCCGO_ERROR "previous|not used"
 	fmt "math"	// ERROR "redeclared|redefinition|incompatible" "imported and not used: \x22math\x22 as fmt"
-	. "math"	// ERROR "imported and not used: \x22math\x22$"
+	. "math"	// GC_ERROR "imported and not used: \x22math\x22$"
 )