MIPS: Clean up code stubs and ensure distinct major keys.

Port r23655 (6327324d)

BUG=
R=machenbach@chromium.org, paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/541513002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23674 ce2b1a6d-e550-0410-aec6-3dcde31c8c00