gh-146227: Fix wrong type in _Py_atomic_load_uint16 in pyatomic_std.h (gh-146229)

Also fix a few related issues in the pyatomic headers:

* Fix _Py_atomic_store_uint_release in pyatomic_msc.h to use __stlr32
  on ARM64 instead of a plain volatile store (which is only relaxed on
  ARM64).

* Add missing _Py_atomic_store_uint_release to pyatomic_gcc.h.

* Fix pseudo-code comment for _Py_atomic_store_ptr_release in
  pyatomic.h.