hwpmc/amd: add LBR v2 detection and filter reservation

[afscoelho_gmail.com]

Summary

Detect LBR v2 on PerfMonV2 CPUs. Reserve the per-CPU branch filter
at allocation, and turn branch recording on and off with the PMCs
that use it. PMC_CAP_LBR is not advertised yet.

Depends on D60568

Test Plan

Built amd64 GENERIC + hwpmc.ko, GENERIC without HWPMC_HOOKS, and arm64.
Ran tests/sys/pmc on AMD Zen 6 (LBR v2).