Clang is a little more picky than gcc on unused variable
warnings, which become errors with -Werror. Clean up
these cases for mips.

BUG=
TEST=

Review URL: https://codereview.chromium.org/11175005
Patch from Akos Palfi <palfia@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12748 ce2b1a6d-e550-0410-aec6-3dcde31c8c00