The tty lists were already pretty similar and there hadn't been any real
need for them to remain distinct for some time. As such, merge to a
single file.
The RISC-V console is preserved. For systems where it doesn't exist, its
presence in /etc/ttys is harmless. The uncommented version of the
ttyv8/XDM line from ttys.amd64 was the one chosen.
Reviewed by: imp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D30256
c76616f496