Add a QOM function hook for target-specific disassembly setup. This
allows removal of the #ifdeffery currently implementing target specific
disas setup from disas.c.

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