Signals: Print backtrace on SIGUSR2

Authored by bgamari.

Description

Signals: Print backtrace on SIGUSR2

This uses the backtrace support introduced in D1196 to provide
backtraces from Haskell processes when SIGUSR2 is thrown.

Test Plan: Need to add a test.

Reviewers: scpmw, simonmar, Tarrasch, austin

Reviewed By: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1197

Details