Comment # 11 on bug 1226512 from Takashi Iwai

Since it's an old machine, it might be to worth to check the shutdown/reboot
type, which is acpi as default.  e.g. try to change to "bios" by running as
root:
  echo -n bios > /sys/kernel/reboot/type

There are other types: kbd, efi, triple and pci.