Add a mechanism to override the detected cpu features.

Restore saved caller FP registers on stub failure and preserve FP registers on NotifyStubFailure.

MIPS: Fix and simplify code aging.

Initialize Date parse cache with SMI instead of double to workaround sharing mutable heap numbers in snapshot.

Fix NotifyStubFailureSaveDoubles generation for ia32.

BUG=280531
LOG=N
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/132193005

git-svn-id: http://v8.googlecode.com/svn/branches/3.21@18552 ce2b1a6d-e550-0410-aec6-3dcde31c8c00