hwpmc: switch in process PMCs on a thread's first run

[afscoelho_gmail.com]

Summary

A new thread starts in fork_exit(), not mi_switch(), so its PMCs are
not loaded until its next context switch. Call the switch-in hook
there.

Depends on D60564

Test Plan

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