atomics: atomic_load_consume_ptr(): Accept pointers to const

Now that all directly-implemented loads (with acquire semantics or not)
accept pointers to const, ensure that atomic_load_consume_ptr(), built
on atomic_load_acq_ptr(), does so too, in the common atomics header as
well as *SAN interceptors.

Reviewed by: kib
Fixes: 5e9a82e898d5 ("atomics: Constify loads")
MFC after: 4 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48497