stand: add comconsole backwards compatibility shim for aarch64

Add a compat shim for the "comconsole" name so that people with a
"console=comconsole" in their loader.conf on aarch64 will continue to
work (though with a warning).

This is only aarch64: it will never be there for amd64 (where comconsole
always means talk to the hardware directly). To do that is too hard.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D39983