target/ppc: Explicitly set 2-NaN propagation rule

Set the 2-NaN propagation rule explicitly in env->fp_status
and env->vec_status.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20241025141254.2141506-9-peter.maydell@linaro.org