target/ppc: Implement vmsumudm instruction

Based on [1] by Lijun Pan <ljp@linux.ibm.com>, which was never merged
into master.

[1]: https://lists.gnu.org/archive/html/qemu-ppc/2020-07/msg00419.html



Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Víctor Colombo <victor.colombo@eldorado.org.br>
Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Message-Id: <20220225210936.1749575-7-matheus.ferst@eldorado.org.br>
Signed-off-by: Cédric Le Goater <clg@kaod.org>