Fix HWPMC interrupt handling in Counting Mode

[mw_semihalf.com]

Summary

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

[bz]

The 0xff should probably a #define somewhere but that's unrelated. I think we've done something similar for the timer code in the past; not sure in which review or tree that ended up.

[mw_semihalf.com]

Replace magic number with macro, as pointed by @bz

[bz]

Thanks for the #define !