Update arm and arm64 counters MD bits.

On arm64 use atomics. Then, both arm and arm64 do not need a critical
section around update. Replace all cpus loop by CPU_FOREACH().
This brings arm and arm64 counter(9) implementation closer to current
amd64, but being more RISC-y, arm* version cannot avoid atomics.

Reported by: Alexandre Martins <alexandre.martins@stormshield.eu>
Reviewed by: andrew
Tested by: Alexandre Martins, andrew
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks