Add sdk builder bot script.

It only builds a subset of the ports, only on Linux, all
architectures/libc/configuration.

BUG=168693
R=sbc@chromium.org

Review URL: https://codereview.chromium.org/11975051

git-svn-id: http://naclports.googlecode.com/svn/trunk/src@668 7dad1e8b-422e-d2af-fbf5-8013b78bd812