atomics: Constify loads: riscv: Convert missed atomic_load_acq_64()

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

(cherry picked from commit 5bd31ad86adb5255e40e7d8ecfddd9e49c7004af)