O Debian 9 caiu suportado por CPUs baseadas em 586 mais antigas.
Uma alternativa possível é instalar um kernel mais novo, com:
sudo apt-get install linux-image-4.7.0-1-686-pae
ou, como você pode inicializar uma VM de 32 bits com um kernel de 64 bits, com um hipervisor que suporte 64 bits:
sudo linux-image-4.7.0-1-amd64-unsigned
Da lista de discussão do Debian em Devel
Last year it was decided to increase the minimum CPU features for the i386 architecture to 686-class in the stretch release cycle. This means dropping support for 586-class and hybrid 586/686 processors[1].(Support for 486-class processors was dropped, somewhat accidentally, in squeeze.)
This was implemented in the Linux kernel packages starting with Linux 4.3, which was uploaded to unstable in December last year.
In case you missed that change, gcc for i386 has recently been changed to target 686-class processors and is generating code that will crash on other processors. Any such systems still running testing or unstable will need to be switched to run stable (jessie).