exit status fixes

In https://github.com/kripken/emscripten/pull/1444 I made a blatant mistake in the exception throwing. I've updated it to _always_ throw the ExitStatus exception as it did previously.

I've also added an EXITSTATUS global that can be used by exit callbacks to determine the exit status, making it so the default exit status print could be removed.

Ran o1, sanity and other.