etc/ttys: merge ttys file down to single file

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