MIPS: Fix register usage after r21822.

The at register is no longer in usage, so changed it to "scratch".

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/335753003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21825 ce2b1a6d-e550-0410-aec6-3dcde31c8c00