Replace OS::MemCopy with memcpy in typed array initialization.

Replace OS::MemCopy with memcpy in typed array initialization.

This is an attempt to fix a weird perf regression on MacOS X 10.6 bot.
Perf regression is not reproducible on MacOS X 10.8.
This difference in memory-copying routines is the only reasonable
difference in the algorithm. We should try and see.

R=bmeurer@chromium.org
BUG=270642
Committed: https://code.google.com/p/v8/source/detail?r=16245

[Dmitry Lomov (no reviews), 2013-08-20 11:08:15]

PTAL

[Benedikt Meurer, 2013-08-20 11:13:20]

LGTM

On a side note: The system memcpy() is pretty much optimal nowadays in nearly
all cases, but we still try to be smarter? :-)

[Dmitry Lomov (no reviews), 2013-08-20 11:15:05]

Committed patchset #2 manually as r16245.