MIPS: Fix register usage in ProfileEntryHookStub

Different register must be used to pass the test on target hardware.

BUG=
TEST=cctest/test-api/SetFunctionEntryHook

Review URL: https://codereview.chromium.org/18368002
Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15432 ce2b1a6d-e550-0410-aec6-3dcde31c8c00