target/ppc: Implement vextsd2q

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Lucas Coutinho <lucas.coutinho@eldorado.org.br>
Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Message-Id: <20220225210936.1749575-9-matheus.ferst@eldorado.org.br>
Signed-off-by: Cédric Le Goater <clg@kaod.org>