hwpmc_armv7: plug memory leak

Free allocated per-CPU structures in the pcpu_fini and finalize methods.

While here, add debug trace entries to these methods.

Reviewed by: jkoshy, andrew
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41269

(cherry picked from commit 9c0a2d522f3a18fc795a7dd6266d2be2aff4b084)