MIPS: Fix DIRECT_API_CALL_NEW and DIRECT_GETTER_CALL_NEW call.

This commit fixes the register usage of DIRECT_API_CALL_NEW
and DIRECT_GETTER_CALL_NEW:

* These functions expect arguments in a0-a1 and not in a1-a2
  as the and DIRECT_API_CALL and DIRECT_GETTER_CALL do.
* Fixes the simulator to expect *_NEW arguments in a0-a1.
* Adds more comment to simulator to better explain the
  register usage.

TEST=cctest/test-api/LoadICFastApi_DirectCall_GCMoveStub,cctest/test-api/SimpleCallback

BUG=

Review URL: https://codereview.chromium.org/15813005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14813 ce2b1a6d-e550-0410-aec6-3dcde31c8c00