target-arm: Implement privileged-execute-never (PXN)

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>

target-arm/cpu.h

@@ -386,6 +386,8 @@ enum arm_features {
     ARM_FEATURE_CACHE_DIRTY_REG, /* 1136/1176 cache dirty status register */
     ARM_FEATURE_CACHE_BLOCK_OPS, /* v6 optional cache block operations */
     ARM_FEATURE_MPIDR, /* has cp15 MPIDR */
+    ARM_FEATURE_PXN, /* has Privileged Execute Never bit */
+    ARM_FEATURE_LPAE, /* has Large Physical Address Extension */
 };
 
 static inline int arm_feature(CPUARMState *env, int feature)