Do not return the constant singleton float -0.0 from UnaryOpStub, because it may be overwritten, on X64 platform.
Review URL: http://codereview.chromium.org/3748007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5631 ce2b1a6d-e550-0410-aec6-3dcde31c8c00