MdePkg Base.h: Add definition for CLANG9 tool chain
    
    BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=1603
    CLANG9 tool chain defines __clang__ macro only,
    doesn't define __GNUC__ macro. But, it uses some same definitions with GCC.
    So, update base definition for CLANG9 tool chain.
    
    Signed-off-by: Liming Gao <liming.gao@intel.com>
    Cc: Michael Kinney <michael.d.kinney@intel.com>
    Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
    Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>