Only implemented on ia32 and x64 for now.  The generic keyed load stub
on arm is falling behind and it is time to fix that, but that will be
a separate change.


Review URL: http://codereview.chromium.org/460142

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3445 ce2b1a6d-e550-0410-aec6-3dcde31c8c00