cpu_loop() never exits, so mark it with QEMU_NORETURN.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-By: Warner Losh <imp@bsdimp.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Message-Id: <20211106113916.544587-1-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>