go/build: use os.MkdirAll to create directories
go/build: use os.MkdirAll to create directories

Replaces "mkdir -p ..." command, as Windows mkdir command
does not support -p flag. Also, is simpler and faster then
launching new process.

R=golang-dev, mattn.jp, adg, bradfitz, rsc
CC=golang-dev
http://codereview.appspot.com/4626087
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go