Expose hs_exit_(rtsFalse) as hs_exit_nowait()

Summary: And document it.  See the docmentation for the reason I want this.

Test Plan: It's an existing API, just exposing it.

Reviewers: bgamari, niteria, austin, erikd

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2531