This renames the method to LookupNumberStringCache() and puts it into
the MacroAssembler in preparation of the NumberToStringStub Hydrogen
conversion.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/23618056

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16806 ce2b1a6d-e550-0410-aec6-3dcde31c8c00