Implement Math.floor stub on ARM. Uses VFP when available.  This is a commit of http://codereview.chromium.org/5075002/ for Martyn Capewell

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5835 ce2b1a6d-e550-0410-aec6-3dcde31c8c00