libpmc, pmcstat: add the lbr qualifier and decode LBR records

[afscoelho_gmail.com]

Summary

Add the "lbr" event qualifier, and print branch records in pmcstat -R.
Add pmc.lbr(3).

Depends on D60570

Test Plan

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

[afscoelho_gmail.com]

Rebase on the updated D60567. No code changes.

[afscoelho_gmail.com]

Rebase only. No code change.