because it's incomplete on x64.

BUG=v8:2808
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/20690002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15893 ce2b1a6d-e550-0410-aec6-3dcde31c8c00