pmccontrol: improve -L with pmu-events

Check if the pmu utils are supported rather than carrying a
machine-dependent #ifdef.

Reviewed by: gnn, ray, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30526

(cherry picked from commit 167cdaa7e30093215a753d4f788d921b1f7c1474)