atomic: Fix the atomic_load_ptr() *SAN interceptor

The interceptor didn't handle a pointer of type "foo * const *" and in
that case we'd get compiler errors 1) an invalid cast to volatile
uintptr_t, and 2) an assignment to a variable of type "foo * const"
(__retptr).

Reported by: mjg
MFC after: 1 week

(cherry picked from commit 08b0c98006b6b9f3722df0ce462f6a4aa8ee06f2)