ArmPkg/UncachedMemoryAllocationLib: map uncached allocations non-executable

The primary use case for UncachedMemoryAllocationLib is non-coherent DMA,
which implies that such regions are not used to fetch instructions from.

So let's map them as non-executable, to avoid creating a security hole
when the rest of the platform may be enforcing strict memory permissions
on ordinary allocations.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>