test: add a test that gccgo failed to compile

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/13632057

test/fixedbugs/bug478.dir/a.go

+// 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.
+
+package p1
+
+type S1 struct{}
+
+func (s S1) f() {}

test/fixedbugs/bug478.dir/b.go

+// 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.
+
+package p2
+
+import "./a"
+
+type S2 struct {
+	p1.S1
+}
+
+func (s S2) f() {}

test/fixedbugs/bug478.go

+// compiledir
+
+// 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.
+
+// Using the same unexported name for a method as a method on an
+// imported embedded type caused a gccgo compilation failure.
+
+package ignored