Note that the name `ICU_SPECIAL_MASK_MODE' isn't pulled out of
nowhere.  If the ICU doesn't support/isn't configured to `special mask
mode', it will automatically block all lower-priority (where
`priority' to the ICU is directly related to the numerical index of
the IRQ vector) interrupts while a higher-priority one is active.
This is generally undesirable.  The option is not enabled by default
because it's not clear that this option is implemented in
clone/integrated chips.