target/arm: Enable FEAT_CSV2_2 for -cpu max

There is no branch prediction in TCG, therefore there is no
need to actually include the context number into the predictor.
Therefore all we need to do is add the state for SCXTNUM_ELx.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220506180242.216785-21-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>