ArmPkg/BaseMemoryLibStm: implement new IsZeroGuid() API function
    
    BaseMemoryLib has recently been extended with an API function
    IsZeroGuid(), so copy the default implementation into BaseMemoryLibStm
    as well.
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
    Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>