Breaks compilation of ARM64.

| 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

BUG=none
LOG=n
TBR=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/458193002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23030 ce2b1a6d-e550-0410-aec6-3dcde31c8c00