disas: QOMify alpha specific disas setup

Move the target_disas() alpha specifics to the CPUClass::disas_set_info()
hook and delete the #ifdef specific code in disas.c.

This also makes monitor_disas() consistent with target_disas(), as
monitor_disas() was missing a set of the BFD (This was an omission from
commit b9bec751).

Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Acked-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Andreas Färber <afaerber@suse.de>