Add support for AMD PerfMonV2 (Family 19h+) core counters, which need
both the per-counter EVSEL enable bit and the global GLOBAL_CTL bit set
to count. Detects PerfMonV2 at init and switches to v2-specific
start/stop/interrupt handlers; older CPUs and L3/DF counters keep using
the classic path unchanged. Adds a read-only sysctl,
kern.hwpmc.amd_perfmon_v2, to report which path is active.