[PowerPC] Move pmc_hook out of critical section

Calling pmc_hook inside a critical section may result in a panic.

This happens when the user callchain is fetched, because it uses
pmap_map_user_ptr, that tries to get the (sleepable) pmap lock when the
needed vsid is not found.

Judging by the implementation in other platforms, intr_irq_handler in
kern/subr_intr.c and what pmc_hook do, it seems safe to move pmc_hook
outside the critical section.

Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D26111