sys: Add cpu_update_pcb hook

This MD function is invoked before dumping register set notes when
writing out a core dump to ensure that the PCB for a given thread is
up to date. This provides a centralized place to update the PCB with
values of the current thread for each arch rather than doing this work
in each register set's get method.

Discussed with: jrtc27
Reviewed by: kib, markj
Sponsored by: AFRL, DARPA
Differential Revision: https://reviews.freebsd.org/D44910