Fix an X64 problem with builtin fixups.  Remove an assert from fixups that is no longer true on all platforms.
Review URL: http://codereview.chromium.org/149332

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2397 ce2b1a6d-e550-0410-aec6-3dcde31c8c00