Move OS::MemCopy and OS::MemMove out of platform to utils

Since both are jitted on some platforms and depend on codegen, they
don't belong to the platform abstraction. At the same time, I can't put
them to codegen.h, as this would introduce cyclic dependencies.

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

Review URL: https://codereview.chromium.org/302563004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21502 ce2b1a6d-e550-0410-aec6-3dcde31c8c00