X87: Clean up code stubs and ensure distinct major keys.

port r23655.

original commit message:

  Clean up code stubs and ensure distinct major keys.

BUG=
R=weiliang.lin@intel.com

Review URL: https://codereview.chromium.org/543013002

Patch from Jing Bao <jing.bao@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23712 ce2b1a6d-e550-0410-aec6-3dcde31c8c00