This enables proper stack unwinding on x64.

Currently this requires V8 to be compiled without snapshot and --gdbjit-full to be enabled.

Original patch by Sanjoy Das (http://codereview.chromium.org/6371011/)

Review URL: http://codereview.chromium.org/6250104

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6575 ce2b1a6d-e550-0410-aec6-3dcde31c8c00