test: add test that gccgo compiled incorrectly

LGTM=minux.ma
R=golang-codereviews, minux.ma
CC=golang-codereviews
https://golang.org/cl/94100045

test/fixedbugs/bug485.go

+// run
+
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Gccgo chose the wrong embedded method when the same type appeared
+// at different levels and the correct choice was not the first
+// appearance of the type in a depth-first search.
+
+package main
+
+type embedded string
+
+func (s embedded) val() string {
+	return string(s)
+}
+
+type A struct {
+	embedded
+}
+
+type B struct {
+	A
+	embedded
+}
+
+func main() {
+	b := &B{
+		A: A{
+			embedded: "a",
+		},
+		embedded: "b",
+	}
+	s := b.val()
+	if s != "b" {
+		panic(s)
+	}
+}