test: add test case that gccgo crashed on

Change-Id: I4d5d40e1ed3f58b2cdecd6248cb25c8ae9a1b9a1
Reviewed-on: https://go-review.googlesource.com/62531Reviewed-by: David Crawshaw <crawshaw@golang.org>

test/fixedbugs/bug503.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 file, due to failing to correctly emit
+// the type descriptor for a named alias.
+
+package p
+
+type entry = struct {
+	a, b, c int
+}
+
+var V entry