Add pushad/popad instructions to assembler.

R=sebmarchand@chromium.org, chrisha@chromium.org

Review URL: https://codereview.appspot.com/136990043

git-svn-id: http://sawbuck.googlecode.com/svn/trunk@2283 15e8cca8-e42c-11de-a347-f34a4f72eb7d