Get the value and decrement it by i

Property definitions

pthreads $ AtomicInt :: get_and_decrement_by
	# Get the value and decrement it by `i`
	fun get_and_decrement_by(i: Int): Int `{
		return __sync_fetch_and_sub(self, i);
	`}
lib/pthreads/pthreads.nit:97,2--100,3