hwpmc: Define full set of Armv8.0 events

Like many of the other encodings here, none of these are actually used
by our tables. However, defining the EVENT_xH names allows them to be
used by the user (e.g. when trying to use an implementation-defined
event that they know about from their core's documentation but we don't)
and allows us to define PMC_EV_ARMV8_LAST appropriately.

Some of these are also used downstream in CheriBSD on Morello.

Reviewed by: andrew, tsoome
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D36926