libpmc: add armv8 pmu-event aliases

These are all "standard microarchitectural events", which in theory are
supported by every ARMv8 processor. In practice, it depends on the
pmu-event definitions being complete and accurate, which they are not
for every processor. Still, these aliases should be functional on the
majority of systems.

PR: 254532
Reported by: emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35550