Serial kernel debugging amd64 on real hardware

[Peter Kay <syllopsium%syllopsium.co.uk@localhost>]

Is this working now? I see there was an issue back in 2015, and that a
minor code change has been made, but I'm still seeing the same

'kgdb waiting... fatal breakpoint trap in supervisor mode'
'trap type 1 code 0.' etc etc
error on boot. This is the same whether on the initial 9.0 release, or
the 202008300420 build (other builds tried too, same issue).

I see the same response on real hardware, and also if I try using
VMWare to emulate it using VMs. Serial comms via tip are fine.

I need to debug real hardware, so using a VM for everything isn't going to work.