build: implement clean.bat

Fixes #2893.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5685047

src/clean.bat

+:: Copyright 2012 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
+
+setlocal
+
+set GOBUILDFAIL=0
+
+go tool dist env -wp >env.bat
+if errorlevel 1 goto fail
+call env.bat
+del env.bat
+echo.
+
+if exist %GOTOOLDIR%\dist.exe goto distok
+echo cannot find %GOTOOLDIR%\dist; nothing to clean
+goto fail
+:distok
+
+"%GOBIN%\go" clean -i std
+%GOTOOLDIR%\dist clean
+
+goto end
+
+:fail
+set GOBUILDFAIL=1
+
+:end
+if x%GOBUILDEXIT%==x1 exit %GOBUILDFAIL%