test: add a test that gccgo failed to compile

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/13451045

test/fixedbugs/bug477.go

+// compile
+
+// Copyright 2013 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.
+
+// Test multiple identical unnamed structs with methods.  This caused
+// a compilation error with gccgo.
+
+package p
+
+type S1 struct{}
+
+func (s S1) M() {}
+
+type S2 struct {
+	F1 struct {
+		S1
+	}
+	F2 struct {
+		S1
+	}
+}
+
+type I interface {
+	M()
+}
+
+func F() {
+	var s2 S2
+	var i1 I = s2.F1
+	var i2 I = s2.F2
+	_, _ = i1, i2
+}