R1:e4f49f080657

When available, use async_signals in Phabricator

Summary: Ref T9640. See D17200 for the analogous change in libphutil.

Test Plan: See D17200.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9640

Differential Revision: https://secure.phabricator.com/D17201