exp/types: don't test importing types if using gccgo

The exp/types packages does not support the gccgo export data
format.  At some point it should, but not yet.

R=gri, bradfitz, r, iant, dsymonds
CC=golang-dev
https://golang.org/cl/6854068

src/pkg/exp/types/gcimporter_test.go

@@ -127,6 +127,10 @@ var importedObjectTests = []struct {
 }
 
 func TestGcImportedTypes(t *testing.T) {
+	// This package does not yet know how to read gccgo export data.
+	if runtime.Compiler == "gccgo" {
+		return
+	}
 	for _, test := range importedObjectTests {
 		s := strings.Split(test.name, ".")
 		if len(s) != 2 {