Replace nonexistent a4 by t0 on mips.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/136093010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19016 ce2b1a6d-e550-0410-aec6-3dcde31c8c00