go.sys/windows: use $GOROOT/src/pkg/syscall/mksyscall_windows.go to generate syscalls

Also zsyscall_windows_386.go and zsyscall_windows_amd64.go
have same contents, so merge them into single file.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/124430043

windows/Makefile

@@ -0,0 +1,12 @@
+# Copyright 2014 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.
+
+all: zsyscall_windows.go
+
+TMP=/tmp/mksyscall_windows
+
+zsyscall_windows.go: syscall_windows.go security_windows.go
+	go build -o $(TMP) $(GOROOT)/src/pkg/syscall/mksyscall_windows.go
+	GOOS=windows $(TMP) $^ | gofmt > $@
+	rm $(TMP)