On Unix we now use negative exit codes in ExitFailure to indicate that a
process exited due to a signal. This patch implements the case for when
a ExitFailure exception propagates out of the top of main (and is
handled by the topHandler).

For a negative ExitFailure code, we try to kill the process using that
signal (the details of that are handled by shutdownHaskellAndSignal from
the RTS). For an exit code outside the valid ranges, we use 0xff.