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/support/StringPrivate.h

@@ -32,12 +32,12 @@ public:
 		return fString._IsShareable();
 	}
 
-	static vint32& DataRefCount(char* data)
+	static int32& DataRefCount(char* data)
 	{
 		return *(((int32 *)data) - 2);
 	}
 
-	vint32& DataRefCount()
+	int32& DataRefCount()
 	{
 		return DataRefCount(Data());
 	}