target/arm: Split out aa32_max_features

Share the code to set AArch32 max features so that we no
longer have code drift between qemu{-system,}-{arm,aarch64}.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220506180242.216785-9-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>