test: add test case that caused a gccgo compiler crash

Change-Id: Icdc980e0dcb5639c49aba5f4f252f33bd207e4fa
Reviewed-on: https://go-review.googlesource.com/c/162617
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

test/fixedbugs/gcc89321.go

+// compile
+
+// Copyright 2019 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.
+
+// https://gcc.gnu.org/PR89321
+// gccgo compiler crash building map literals with a zero-sized value type.
+
+package p
+
+type M map[byte]struct{}
+
+var (
+	M1 = M{1: {}, 2: {}, 3: {}}
+	M2 = M{1: {}, 2: {}}
+)