When linking against libpthreads, raise(3) can behave strangely on
some platforms (OpenBSD at least, maybe other BSDs, not Linux). So use
pthread_kill() to generate signals when running the threaded rts,
instead of raise(), when System.Posix.Signals.raiseSignal is called.

raiseSignal will call genericRaise, in the rts, which knows which
function to use.