libpmc: Fix the L3 counters for AMD Zen 1-4

On AMD processors libpmc was using the topic field (based on filename) to
determine the counter's subclass. Unfortunately, the JSON definitions for
AMD Zen 1-4 have the L3 counters in files shared with other counters.

This change has libpmc to use the pmu field (which is derived from the Unit
field in JSON) to determine the correct counter subclass.

Reviewed by: mhorne
MFC after: 2 weeks
Sponsored by: Netflix
Pull Request: https://github.com/freebsd/freebsd-src/pull/1984

(cherry picked from commit c215eef345501ce7dda374909b3195d9d69a4e9f)