hwpmc_arm64: accept raw event codes for PMC_OP_PMCALLOCATE

Make it possible to specify event codes without an offset of
PMC_EV_ARMV8_FIRST, by setting a machine-dependent flag. This is
required to make use of event definitions from pmu-events.

Reviewed by: ray (slightly earlier version)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30602