MIPS: Improve runtime detection and compatibility wrt arch. revisions.

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

Review URL: https://codereview.chromium.org/618193005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24486 ce2b1a6d-e550-0410-aec6-3dcde31c8c00