hw/cpu/a9mpcore: Build it only once

hw/cpu/a9mpcore: Build it only once

hw/cpu/a9mpcore.c doesn't require "cpu.h" anymore.
By removing it, the unit become target agnostic:
we can build it once. Update meson.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20240118200643.29037-13-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>