target/arm: Update qemu-system-arm -cpu max to cortex-a57

Instead of starting with cortex-a15 and adding v8 features to
a v7 cpu, begin with a v8 cpu stripped of its aarch64 features.
This fixes the long-standing to-do where we only enabled v8
features for user-only.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220506180242.216785-7-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>