GHC installs signal handlers in runGhc/runGhcT to handle ^C but it
never uninstalls them.
It can be an issue, especially when using GHC as a library.

Test Plan: validate

Reviewers: bgamari, erikd, austin, simonmar

Reviewed By: bgamari, simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2633

GHC Trac Issues: #4162