accel: Make AccelCPUClass structure target-agnostic

accel: Make AccelCPUClass structure target-agnostic

Move the target-agnostic parts of "accel/accel-cpu-target.h"
to "accel/accel-cpu.h".

Doing so we need to include missing "hw/core/cpu.h" header
in "accel/accel-cpu.h" otherwise we get:

  include/accel/accel-cpu-target.h:39:28: error: unknown type name 'CPUClass'
     39 |     void (*cpu_class_init)(CPUClass *cc);
        |                            ^

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20250417165430.58213-7-philmd@linaro.org>