gatomic: Use g(u)intptr where appropriate

Open Alexander Richardson requested to merge arichardson/glib:atomic-uintptr into main

This is required for CHERI systems such as Arm Morello, where gsize is not large enough to hold a pointer. For all other architectures this should not result in any functional changes since gsize is already the same size (and I believe for most architectures even the same type) as guintptr.

This is fully ABI compatible and should also be fully API compatible except for some rather unlikely examples such as someone happening to use something like decltype(g_atomic_pointer_add(&a, 0)) in a C++ context where it changes name mangling of some function.

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