target/ppc: use ext32u and deposit in do_vx_vmulhw_i64

Fixes: 29e9dfcf ("target/ppc: vmulh* instructions without helpers")
Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220304175156.2012315-3-matheus.ferst@eldorado.org.br>
Signed-off-by: Cédric Le Goater <clg@kaod.org>