tweak of Math.abs in its x64 stub

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/22285003

Patch from Weiliang Lin <weiliang.lin@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16113 ce2b1a6d-e550-0410-aec6-3dcde31c8c00