Provide BitField64 utility class.

R=svenpanne@chromium.org
TEST=cctest/test-conversions/BitField64

Review URL: https://codereview.chromium.org/14643004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14681 ce2b1a6d-e550-0410-aec6-3dcde31c8c00