toString() on generator functions prints with function*

This CL adds a %FunctionIsGenerator runtime function, and uses it in the
function toString() implementation.

R=mstarzinger@chromium.org
BUG=v8:2355
TEST=mjsunit/harmony/generators-runtime

Review URL: https://codereview.chromium.org/14912002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14544 ce2b1a6d-e550-0410-aec6-3dcde31c8c00