test: add test that gccgo miscompiled

Change-Id: Ic3f006f86a86de628e14b107f88a5923ea856a58
Reviewed-on: https://go-review.googlesource.com/18093Reviewed-by: David Symonds <dsymonds@golang.org>

test/fixedbugs/bug497.go

+// run
+
+// Copyright 2015 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 used to miscompile passing a global variable with a
+// zero-sized type to a function.
+
+package main
+
+type T struct {
+	field s
+}
+
+type s struct{}
+
+var X T
+
+func F(_ T, c interface{}) int {
+	return len(c.(string))
+}
+
+func main() {
+	if v := F(X, "hi"); v != 2 {
+		panic(v)
+	}
+}