We need to call GHC.TopHandler.runIOFastExit instead of
GHC.TopHandler.runIO.  Recent fixes to the shutdown code have meant
that when a thread invokes shutdownHaskellAndExit(), other main
threads get a chance to exit (as they should), but this means that we
might have a race between the child thread trying to exit the program
and the main thread doing the same.  In the case of GHCi, if we're
running an interpreted computation that needs to exit (as some tests
do), then we really want this child thread to exit the program rather
than the main thread.