Summary:
When we call runHandlers, we must pass it a ForeignPtr. To ensure that
this happens, we introduce a wrapper that receives a plain Ptr and
converts it into a ForeignPtr. Then we adjust startSignalHandlers in
rts/posix/Signals.c to call the wrapper instead of calling runHandlers
directly.

Reviewers: hvr, austin, rwbarton, simonmar

Reviewed By: austin, simonmar

Subscribers: simonmar, thomie, carter

Differential Revision: https://phabricator.haskell.org/D515

GHC Trac Issues: #9817

(cherry picked from commit 7ca5bb09)