Harmony: implement Math.sign.

R=dslomov@chromium.org
BUG=v8:2938

Review URL: https://codereview.chromium.org/28723002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17279 ce2b1a6d-e550-0410-aec6-3dcde31c8c00