Implement Math.log1p using port from fdlibm.

Port contributed by Raymond Toy <rtoy@google.com>.

R=rtoy@chromium.org
LOG=N
BUG=v8:3481

Review URL: https://codereview.chromium.org/457643002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23082 ce2b1a6d-e550-0410-aec6-3dcde31c8c00