It's not strictly necessary, because contrary to g_atomic_pointer_get()
and g_atomic_pointer_compare_and_exchange(), glib's variant for the
setter is mostly fine.
Still, reimplement it, because we use typeof() eagerly and can thus add
more static checks than glib.
(cherry picked from commit 7c60e984b6)
(cherry picked from commit 6ded463f36)
(cherry picked from commit 976b358be6)
(cherry picked from commit 296a770a85)
(cherry picked from commit 32c81a29d5)
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