MIPS: Add support for arch. revision 6 to mips32 port.

MIPS: Add support for arch. revision 6 to mips32 port.

Additional summary:
 - Introduce fp64 fpu mode into mips32 port required for r6.
 - Implement runtime detections for fpu mode and arch. revision to preserve
   compatibility with previous architecture revisions.

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

Review URL: https://codereview.chromium.org/453043002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23028 ce2b1a6d-e550-0410-aec6-3dcde31c8c00