bhyve: refactor gdbstub to enable single-stepping on AMD CPUs

This patch refactors the existing Intel-specific single-stepping
mechanism in bhyve's GDB stub to work with both AMD and Intel CPUs.

Reviewed by: jhb
Sponsored by: Google, Inc. (GSoC 2022)
Differential Revision: https://reviews.freebsd.org/D42298

(cherry picked from commit ca96a942cafb58476e10e887240e594e7923a6e8)