target-s390x: implement miscellaneous-instruction-extensions facility

RISBGN is the same as RISBG, but without setting the condition code.
CLT and CLGT are the same as CLRT and CLGRT, but using memory for the
second operand.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Alexander Graf <agraf@suse.de>