You wouldn't.
Hint: How to read the value and modify it in arbitrary way and have that in single, atomic operation? Just reading or writing the value doesn't give anything useful, or at least I can't think of any use for that.
Even if you would only read or modify it, even the modify might not be atomic operation (it isn't with PPC).
If the code is m68k only, and if the code only does atomic read or update operation, then it would work. But like I said, such conditions aren't met often.
