hwpmc: fix multipart sample accounting

[afscoelho_gmail.com]

Summary

Fix how multipart samples (IBS) are stored:

• Clear and count the whole 8-byte header (two slots on 32-bit).
• Count the payload when the callchain depth is 1.
• Do not reserve an extra slot for the user callchain.
• Use the real PC, not the header, for profil(2).
• Make sure a sample slot can hold the largest payload.

Add <sys/pmclog.h> helpers to find the callchain in a multipart record.

Test Plan

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