Merged r16772, r18000, r18030, r18298, r18319 into 3.20 branch.

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/136333003

git-svn-id: https://v8.googlecode.com/svn/branches/3.20@18555 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Merge remote-tracking branch 'upstream/3.20' into 3.20