Turn the NumberToStringStub into a hydrogen stub.

This adds a BuildLookupNumberStringCache() in Hydrogen, which will
be used by the hydrogen version of StringAddStub, in addition to the
hydrogen version of NumberToStringStub.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/23726041

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16874 ce2b1a6d-e550-0410-aec6-3dcde31c8c00