test: add a test that causes gccgo to get a failure at link time

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/13788043

test/fixedbugs/bug479.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 p
+
+import "unsafe"
+
+type S2 struct {}
+
+const C = unsafe.Sizeof(S2{})
+
+type S1 struct {
+	S2
+}

test/fixedbugs/bug479.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 main
+
+import "./a"
+
+type S3 struct {
+	p.S1
+}
+
+func main() {
+	var i interface{} = S3{}
+	_ = i
+}

test/fixedbugs/bug479.go

+// rundir
+
+// 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.
+
+// Gccgo was not consistent in deciding how to compare a struct type
+// for equality, leading to an undefined symbol at link time.
+
+package ignored