kern: run the PMC context-switch-in hook in fork_exit

[afscoelho_gmail.com]

Summary

A new thread reaches the CPU for the first time through fork_exit()
instead of sched_switch(), so the context-switch-in hook that
sched_switch() runs is skipped. An inherited PMC is then not
programmed onto the hardware until the child's first real context
switch, and hwpmc does not count the work of a short-lived child.

Run the hook from fork_exit() for a process that uses PMCs. This
mirrors the mi_switch() path that fork_exit() already emulates.

Sponsored by: AMD