test: add test for gccgo compiler crash

Gccgo crashed compiling a function that returned multiple zero-sized values.

Change-Id: I499112cc310e4a4f649962f4d2bc9fee95dee1b6
Reviewed-on: https://go-review.googlesource.com/38772Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/fixedbugs/gcc80226.go

+// compile
+
+// Copyright 2017 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.
+
+// The gccgo compiler crashed while compiling a function that returned
+// multiple zero-sized structs.
+// https://gcc.gnu.org/PR80226.
+
+package p
+
+type S struct{}
+
+func F() (S, S) {
+	return S{}, S{}
+}