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Update the debug monitor handling to work after userspace has started

Description

Update the debug monitor handling to work after userspace has started

The debug monitor register state is now stored in a struct and updated
when required. Currently there is only a kernel state, however a
per-process state will be added in a future change.

Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D22128