test: add test case that gccgo failed to link

Gccgo creates a struct to hold the arguments for the deferred
function.  In this example the struct holds a type defined in a
different package.  The bug was that gccgo tried to create an equality
function for this struct, and it implemented that function by calling
the equality function for the type defined in the other package.
Since that type is not exported, the reference to the equality
function failed at link time.  Normally it is impossible for a struct
to directly contain a member that is an unexported type from another
package, but in this specific case it was possible.  Fixed in gccgo
with https://codereview.appspot.com/183500043 .

Change-Id: I8ec3a33631225b9ac2a4ac060cb4d10b4635e60b
Reviewed-on: https://go-review.googlesource.com/1690Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

test/fixedbugs/bug492.dir/a.go

+// Copyright 2014 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 a
+
+type s struct {
+	s string
+}
+
+func F1(s s) {
+}
+
+func F2() s {
+	return s{""}
+}

test/fixedbugs/bug492.dir/b.go

+// Copyright 2014 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"
+
+func main() {
+	defer a.F1(a.F2())
+}

test/fixedbugs/bug492.go

+// rundir
+
+// Copyright 2014 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 case that gccgo failed to link.
+
+package ignored