test: add test that caused a gccgo compilation failure

Updates #4734.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/7228079

test/fixedbugs/issue4734.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.
+
+// Caused gccgo to emit multiple definitions of the same symbol.
+
+package p
+
+type S1 struct{}
+
+func (s *S1) M() {}
+
+type S2 struct {
+	F struct{ *S1 }
+}
+
+func F() {
+	_ = struct{ *S1 }{}
+}