sys/atomic_utils: Use volatile pointers
sys/atomic_utils: Use volatile pointers

Mark all pointers in the atomic utils API as `volatile` for the following
reasons:

1. The guarantees provided by `volatile` is a subset of the guarantees
   provided by the atomic utils. This makes this fact more obvious.
2. Casting `volatile` off is undefined behavior. In order to be able to use
   the atomic utils on memory mapped IO (which needs the `volatile` qualifier)
   without resorting to undefined behavior, this API change is needed.

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