test: add test case for which gccgo generated incorrect GC info

Change-Id: I5e33db9e63f70706882b85ab124a48509797b05a
Reviewed-on: https://go-review.googlesource.com/33790
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/bug500.go

+// run
+
+// Copyright 2016 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 generated incorrect GC info when a global variable was
+// initialized to a slice of a value containing pointers.  The initial
+// backing array for the slice was allocated in the .data section,
+// which is fine, but the backing array was not registered as a GC
+// root.
+
+package main
+
+import (
+	"runtime"
+)
+
+type s struct {
+	str string
+}
+
+var a = []struct {
+	str string
+}{
+	{""},
+}
+
+var b = "b"
+var c = "c"
+
+func init() {
+	a[0].str = b + c
+}
+
+func main() {
+	runtime.GC()
+	if a[0].str != b + c {
+		panic(a[0].str)
+	}
+}