Acriave test-hashing in SCons and buildbot runs.
Small cleanups in root register handling on ARM and MIPS.
Review URL: http://codereview.chromium.org/9110029

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10351 ce2b1a6d-e550-0410-aec6-3dcde31c8c00