Decrement the value and get the new one

Property definitions

pthreads $ AtomicInt :: decrement_and_get
	# Decrement the value and get the new one
	fun decrement_and_get: Int `{
		return __sync_sub_and_fetch(self,1);
	`}
lib/pthreads/pthreads.nit:127,2--130,3