target/arm: Replace ARM_FEATURE_PXN with ID_MMFR0.VMSA check

The ARM_FEATURE_PXN bit indicates whether the CPU supports the PXN
bit in short-descriptor translation table format descriptors.  This
is indicated by ID_MMFR0.VMSA being at least 0b0100.  Replace the
feature bit with an ID register check, in line with our preference
for ID register checks over feature bits.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200910173855.4068-2-peter.maydell@linaro.org