libpmc: enable pmu_utils on arm64

This allows supported libpmc to query/select from the pmu-events table,
which may have a more complete set of events than what we define
manually. A future update to these definitions should greatly improve
this support. The alias table is empty for now, until this future import
is complete.

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Reviewed by: ray (slightly earlier version)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30603