Increase number of available major keys.

This also adds a static assert checking that we do not
exceed the available number of major keys. The safepoint
table offset is now max 2^24 instead of 2^25.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/110183003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18284 ce2b1a6d-e550-0410-aec6-3dcde31c8c00