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/os/support/String.h

@@ -396,8 +396,8 @@ private:
 								int32 withLength);
 
 private:
-			vint32& 		_ReferenceCount();
-			const vint32& 	_ReferenceCount() const;
+			int32& 			_ReferenceCount();
+			const int32& 	_ReferenceCount() const;
 			bool			_IsShareable() const;
 			void			_FreePrivateData();