system/cpus: Add cpu_pause() function

system/cpus: Add cpu_pause() function

This factors the CPU pause function from pause_all_vcpus() into a
new cpu_pause() function, similarly to cpu_resume(). cpu_resume()
is moved to keep it next to cpu_pause().

Cc: Philippe Mathieu-Daudé <philmd@linaro.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-ID: <20240712120247.477133-17-npiggin@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>