Fix HWPMC interrupt handling in Counting Mode

Additionally:

• Fix support for Cycle Counter (evsel == 0xFF)
• Stop and mask interrupts from all counters on init and finish

Submitted by: Michal Mazur <mkm@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield, Netgate
Differential revision: https://reviews.freebsd.org/D10910