Move the target_disas() s390 specifics to the CPUClass::disas_set_info()
hook and delete the #ifdef specific code in disas.c.

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