Return int64_t instead of intptr_t for RootRegisterDelta and support x32 as well.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/236753002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20863 ce2b1a6d-e550-0410-aec6-3dcde31c8c00