kernel: atomic_*() functions rework

* No need for the atomically changed variables to be declared as
   volatile.
 * Drop support for atomically getting and setting unaligned data.
 * Introduce atomic_get_and_set[64]() which works the same as
   atomic_set[64]() used to. atomic_set[64]() does not return the
   previous value anymore.

headers/private/shared/WeakReferenceable.h

@@ -30,7 +30,7 @@ public:
 			void				GetUnchecked();
 
 private:
-			vint32				fUseCount;
+			int32				fUseCount;
 			BWeakReferenceable*	fObject;
 };