libpmc: make pmc_pmu_pmcallocate() machine-independent

Have it call the platform-dependent version. For better layering, move
the reset logic inside the new function. This is mainly to facilitate an
upcoming change.

Reviewed by: jkoshy
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40752