windows: make "go generate" command work on any OS

See issue golang/go#16368 for details.

Updates golang/go#16368

Change-Id: Ic3294385a350aee41887f59cbcf5b0af7742ff3b
Reviewed-on: https://go-review.googlesource.com/24952
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

windows/registry/mksyscall.go

@@ -0,0 +1,7 @@
+// 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.
+
+package registry
+
+//go:generate go run $GOROOT/src/syscall/mksyscall_windows.go -output zsyscall_windows.go syscall.go