cpu: Add generic cpu_list()

cpu: Add generic cpu_list()

Add generic cpu_list() to replace the individual target's implementation
in the subsequent commits. Currently, there are 3 targets with no cpu_list()
implementation: microblaze and nios2. With this applied, those two targets
switch to the generic cpu_list().

[gshan@gshan q]$ ./build/qemu-system-microblaze -cpu ?
Available CPUs:
  microblaze-cpu

[gshan@gshan q]$ ./build/qemu-system-nios2 -cpu ?
Available CPUs:
  nios2-cpu

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20231114235628.534334-7-gshan@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>