Use fast in-situ test for gate A20 being set, to cut down on the

Use fast in-situ test for gate A20 being set, to cut down on the
number of (potentially very slow) gateA20_set operations.

Die with a fatal error if we are unable to set gate A20; if this fails
then we are bound to experience memory corruption at a later stage,
and I'd prefer to pick it up early.