target-s390x: optimize (negative-) abs computation

Now that movcond exists, it's easy to write (negative-) absolute value
using TCG code instead of an helper.

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