cp is handled by the register allocator and doesn't necessarily hold the
context pointer on arm64.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/695323005

Cr-Commit-Position: refs/heads/master@{#25209}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25209 ce2b1a6d-e550-0410-aec6-3dcde31c8c00