arm64 hwpmc: Support restricting counters to user or kernel mode.

Support the "usr" and "os" qualifiers on arm64 events to restrict
event counting to either usermode or the kernel, respectively. If
neither qualifier is given, events are counted in both.

Reviewed by: emaste
Sponsored by: University of Cambridge, Google, Inc.
Differential Revision: https://reviews.freebsd.org/D34527