MIPS: Faster memcpy.

BUG=
R=jkummerow@chromium.org, plind44@gmail.com

Review URL: https://codereview.chromium.org/104353002

Patch from yuyin QQ <xwafish@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18274 ce2b1a6d-e550-0410-aec6-3dcde31c8c00