MIPS: Make the mips port build cleanly with clang.

Port r13583 (b39b116b) and also fix a couple mips-specific warnings.

BUG=

Review URL: https://codereview.chromium.org/13321002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14100 ce2b1a6d-e550-0410-aec6-3dcde31c8c00