Small tweaks to make X64 compliant.
A few UNIMPLEMENTED left, but most empty functions call Abort to bail out.

Review URL: http://codereview.chromium.org/6201006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6297 ce2b1a6d-e550-0410-aec6-3dcde31c8c00