windows: add empty.s to fix build with Go 1.11 and earlier

CL 199519 broke the build with Go 1.11. Even though this is not an
officially supported release anymore, people still seem to rely on it
and the fix is easy enough and won't break anyone using newer, supported
Go versions.

Change-Id: I632d6840bc1f49c01c372727c9a55d2daf98f4a8
Reviewed-on: https://go-review.googlesource.com/c/sys/+/206357
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Author: tklauser

windows/empty.s

@@ -0,0 +1,8 @@
+// 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.
+
+// +build !go1.12
+
+// This file is here to allow bodyless functions with go:linkname for Go 1.11
+// and earlier (see https://golang.org/issue/23311).