test: add test that caused gccgo compiler crash

Updates #21253

Change-Id: Iece71a27207b578618cafb378dac2362517363d0
Reviewed-on: https://go-review.googlesource.com/52531
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/issue21253.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.
+
+// Gccgo crashed compiling this code due to failing to finalize
+// interfaces in the right order.
+
+package p
+
+type s1 struct {
+	f m
+	I
+}
+
+type m interface {
+	Mm(*s2)
+}
+
+type s2 struct {
+	*s1
+}
+
+type I interface {
+	MI()
+}