Improve implementation of `sel4-atomic-ptr`

[nspin]

Perhaps by leaning more on core::sync::atomic instead of using intrinsics.

See https://github.com/Amanieu/atomic-rs/tree/master for inspiration.

[nspin]

Upon further investigation:

Leaning on core::sync::atomic won't work because core::sync::atomic::Atomic* are implemented using UnsafeCell, which isn't suitable for shared memory. See this explanation for why not.