Replace some calls to MemCopy with small constant sizes with memcpy

Replace some calls to MemCopy with small constant sizes with memcpy

MemCopy is only meant for variable size, large (>64bytes) copies,
otherwise, it's probably slower than memcpy due to the call
overhead and the compiler can't optimize it away.

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

Committed: https://code.google.com/p/v8/source/detail?r=21519

[Jakob Kummerow, 2014-05-27 11:54:34]

Yeah, using MemCopy for these was kinda stupid. Should be CopyWords, if we
wanted to stick with a hand-rolled solution, but memcpy() is fine too.

LGTM.

[jochen (gone - plz use gerrit), 2014-05-27 12:02:56]

Committed patchset #1 manually as r21519 (presubmit successful).