gc: disallow map/func equality via interface comparison

Missed when I removed direct map/func equality.

R=ken2
CC=golang-dev
https://golang.org/cl/5452052
diff --git a/src/pkg/reflect/all_test.go b/src/pkg/reflect/all_test.go
index a9e5d1c..e432601 100644
--- a/src/pkg/reflect/all_test.go
+++ b/src/pkg/reflect/all_test.go
@@ -468,8 +468,8 @@
 func TestFunctionValue(t *testing.T) {
 	var x interface{} = func() {}
 	v := ValueOf(x)
-	if v.Interface() != v.Interface() || v.Interface() != x {
-		t.Fatalf("TestFunction != itself")
+	if fmt.Sprint(v.Interface()) != fmt.Sprint(x) {
+		t.Fatalf("TestFunction returned wrong pointer")
 	}
 	assert(t, v.Type().String(), "func()")
 }