windows: Add windows build tag to asm.s

This allows linux binaries to have bazel-style dependencies on x/sys/windows.

Change-Id: I248fe62e045705f409e5d96842ed52d0764ae68f
Reviewed-on: https://go-review.googlesource.com/21081
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

windows/asm.s

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build windows
+
 #include "textflag.h"
 
 TEXT ·use(SB),NOSPLIT,$0