Kernel branch records leak kernel addresses. If an LBR PMC can record
kernel branches, require PRIV_PMC_SYSTEM and refuse it in a jail.
User-only LBR keeps the normal rules.

LBR samples need a log file, so require one.

Depends on D60567

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