Most of ARM machines display their CPU when QEMU list the available
machines (-M help). Some machines do not. Fix to unify the help
output.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210131184449.382425-7-f4bug@amsat.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>