target/s390x: Use a single return for helper_divs64/u64

target/s390x: Use a single return for helper_divs64/u64

Pack the quotient and remainder into a single Int128.
Use the divu128 primitive to remove the cpu_abort on
32-bit hosts.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
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v2: Extended div test case to cover these insns.