Fix unwinding of C -> Haskell FFI calls with -threaded (2nd try)

Summary:
See the new note.
This should fix cb5c2fe8 enough
to unbreak Windows and OS X builds.

Test Plan: manual testing with patched gdb

Reviewers: bgamari, simonmar, erikd

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4694