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 with all currently supported architectures
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.

Helps: #2842