hwpmc: initialize arm64 counter/interrupt state

Performance counters and overflow interrupts are assumed to be disabled
by default, but this is not guaranteed. Ensure we disable both during
per-cpu initialization, before enabling the PMU. Otherwise, some systems
(such as the Ampere eMAG) would experience an interrupt storm upon
loading the hwpmc module.

Reviewed by: br
MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32854