hwpmc: log sample drops

[afscoelho_gmail.com]

Summary

Log drop counts and reasons for each PMC and CPU. Set PMC_VERSION_MINOR to 4 and update log readers.

Known limit: with a full log, 9 of 2,001 throttle drops are absent. No correction is included.

Sponsored by: AMD

Test Plan

GENERIC with INVARIANTS on AMD, 160 CPUs.
58 PMC tests pass, with no failures or skips.
Full stack: 59 tests, 200 start/stop cycles, and 40 active unloads pass.
All prefixes build. Old/new log checks pass.