ArmPkg/CompilerIntrinsicsLib: use Clang-compatible 'weak' attribute
    
    Clang does not like separate definitions for the __alias__ and the
    __weak__ attributes, so merge the definitions into one.
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
    Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>