go.sys: copy files from syscall package to go.sys/{plan9,windows,unix}

This CL copies to each package of go.sys the files from syscall it will need.
Different directories have different files, but these:
        mkall.sh
        str.go
        syscall.go
        mksyscall.pl
        race.go
        race0.go
        syscall_test.go
are copied to all three.
No changes yet, these are just copies. They are not ready to use yet:
package names are wrong, for starters. But this clean copy will make
it easier to follow the changes as the packages are enabled.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/126960043

windows/mkall_windows.bat

@@ -0,0 +1,21 @@
+:: Copyright 2013 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.
+@echo off
+
+if exist mkall.sh goto dirok
+echo mkall_windows.bat must be run from src\pkg\syscall directory
+goto :end
+:dirok
+
+if "%1"=="386" goto :paramok
+if "%1"=="amd64" goto :paramok
+echo parameters must be 386 or amd64
+goto :end
+:paramok
+
+go build mksyscall_windows.go
+.\mksyscall_windows syscall_windows.go security_windows.go syscall_windows_%1.go |gofmt >zsyscall_windows_%1.go
+del mksyscall_windows.exe
+
+:end