test: add test that gccgo failed to compile

Updates #28601

Change-Id: I734fc5ded153126d384f0df912ecd4d208005e49
Reviewed-on: https://go-review.googlesource.com/c/147537
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

test/fixedbugs/issue28601.go

+// compile
+
+// Copyright 2018 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.
+
+// Failed to compile with gccgo.
+
+package p
+
+import "unsafe"
+
+const w int = int(unsafe.Sizeof(0))
+
+var a [w]byte