test: add a test that caused gccgo to crash

R=golang-dev
CC=golang-dev
https://golang.org/cl/6849129

test/fixedbugs/bug471.go

+// compile
+
+// Copyright 2012 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.
+
+// Caused an internal compiler error in gccgo.
+
+package p
+
+type C chan struct{}
+
+func (c C) F() {
+	select {
+	case c <- struct{}{}:
+	default:
+	}
+}