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

Review URL: https://codereview.chromium.org/306453005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21519 ce2b1a6d-e550-0410-aec6-3dcde31c8c00