target/s390x: implement PACK UNICODE

Use a common helper with PACK ASCII as the differences are limited to
the stride of the source operand.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Message-Id: <20170531220129.27724-25-aurelien@aurel32.net>
Signed-off-by: Richard Henderson <rth@twiddle.net>
