libpmc: Allow specifying explicit EVENT_xxH events on armv7 and arm64

This is useful for processors where we don't have an event table; in
those cases we default to a Cortex A8 (armv7) or Cortex A53 (arm64) in
order to attempt to provide something useful, but you're then limited to
the counters in those tables, some of which may also not be implemented
(e.g. LD/ST_RETIRED are no longer implemented in more recent cores,
replaced by LD/ST_SPEC).

Adding the raw EVENT_xxH event lists to each table ensures that you can
always request the exact events you want, regardless of what has been
detected or is known.

Reviewed by: mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D33805