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/24952Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

windows/mksyscall.go

+// Copyright 2009 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 windows
+
+//go:generate go run $GOROOT/src/syscall/mksyscall_windows.go -output zsyscall_windows.go eventlog.go service.go syscall_windows.go security_windows.go

windows/registry/mksyscall.go

+// 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

windows/registry/syscall.go

@@ -8,8 +8,6 @@ package registry
 
 import "syscall"
 
-//go:generate go run $GOROOT/src/syscall/mksyscall_windows.go -output zsyscall_windows.go syscall.go
-
 const (
 	_REG_OPTION_NON_VOLATILE = 0
 

windows/syscall_windows.go

@@ -14,8 +14,6 @@ import (
 	"unsafe"
 )
 
-//go:generate go run $GOROOT/src/syscall/mksyscall_windows.go -output zsyscall_windows.go eventlog.go service.go syscall_windows.go security_windows.go
-
 type Handle uintptr
 
 const InvalidHandle = ^Handle(0)