hwpmc: Permit the minimum sampling count to be set as a sysctl.

A rarely occurring event (e.g. an event that occurs less than 1000
times during execution of a program) may require a lower minimum
threshold than 1000. Replace the hardcoded 1000 with a sysctl that
the administrator can use to permit smaller sampling count values.

Reviewed by: mhorne, mav
Sponsored by: University of Cambridge, Google, Inc.
Differential Revision: https://reviews.freebsd.org/D35400