target-s390x: implement load-and-trap facility

At the same time move the trap code from op_ct into gen_trap and use it
for all new functions. The value needs to be stored back to register
before the exception, but also before the brcond (as we don't use
temp locals). That's why we can't use wout helper.

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