Implement the privileged-execute-never (PXN) translation table bit.
It is implementation-defined whether this is implemented, so we give
it its own ARM_FEATURE_ flag. LPAE requires PXN, so add also an
LPAE feature flag and the implication logic, as a placeholder
for actually implementing LPAE at a later date.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>