Decrement by i and get the new value

Property definitions

pthreads $ AtomicInt :: decrement_by_and_get
	# Decrement by `i` and get the new value
	fun decrement_by_and_get(i: Int): Int `{
		return __sync_sub_and_fetch(self, i);
	`}
lib/pthreads/pthreads.nit:117,2--120,3