MdePkg/Uefi ARM: only support native ARM/Thumb images

MdePkg/Uefi ARM: only support native ARM/Thumb images

The ARM calling convention is fundamentally incompatible with EBC,
and having a cross compatible machine type identical to the native
type does not make a lot of sense either. So restrict the compatible
machine type for ARM to EFI_IMAGE_MACHINE_ARMTHUMB_MIXED, and remove
the cross compatible.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>