libpmc: always generate libpmc_events.c

The jevents build tool will create an empty table if it doesn't find any
events, so we can remove the extra $MACHINE_CPUARCH checks.

Reviewed by: gnn, ray, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30531