only enable mips assembly for old 32 bit abi.  new 32 bit abi and 64 bit bit able remove t4 to t7 and add a4 to a7
BUG=337
TESTED=untested
R=tpsiaki@google.com

Review URL: https://webrtc-codereview.appspot.com/20769005

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1020 16f28f9a-4ce2-e073-06de-1de4eb20be90