v8: remove optimization switches

Remove compiler switches from $(TOPLEVEL)/deps/v8/build/common.gypi,
we set them globally in $(TOPLEVEL)/common.gypi.

Commit 7b4d95a introduced the switches again, resulting in V8 getting
built without any optimizations.

This commit is essentially a rehash of commit 4b8629d.
Merge remote-tracking branch 'origin/v0.8'
buffer: floating point improvements

Improvements:
* floating point operations are approx 3x's faster
* NaN/Infinity assignation more closely follows d8's implementation
* all read/write on floating point now done in C, so no more need for
  lib/buffer_ieee754.js