MdePkg: Arm/AArch64 - filter #pragma pack() when __ASSEMBLER__

MdePkg: Arm/AArch64 - filter #pragma pack() when __ASSEMBLER__

clang, when used as a preprocessor for dtc, does not discard #pragma
statements although -x assembler-with-cpp is specified. This causes dtc
to barf at a #pragma pack() statement that is already filtered out for
__GNUC__. So add a check to also filter this out if __ASSEMBLER__.

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