vmm: enable software breakpoints for AMD CPUs

This patch adds support for software breakpoint vmexits on AMD SVM.
It implements the VM_CAP_BPT_EXIT used to enable software breakpoints.
When enabled, breakpoint vmexits are passed to userspace where they
are handled by the GDB stub.

Reviewed by: jhb
Sponsored by: Google, Inc. (GSoC 2022)
Differential Revision: https://reviews.freebsd.org/D42295