libpmc, pmcstat: harden multipart sample consumers

[afscoelho_gmail.com]

Summary

Skip the multipart header and payload when reading callchains, and
check record lengths before use. Add ATF tests.

Depends on D60563

Test Plan

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