target/ppc: Implement Vector Compare Greater Than Quadword

Implement the following PowerISA v3.1 instructions:
vcmpgtsq: Vector Compare Greater Than Signed Quadword
vcmpgtuq: Vector Compare Greater Than Unsigned Quadword

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Message-Id: <20220225210936.1749575-13-matheus.ferst@eldorado.org.br>
Signed-off-by: Cédric Le Goater <clg@kaod.org>