MdePkg BaseIoLibIntrinsic: Remove __inline__ attribute for IO functions
    
    BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=1603
    __inline__ has no functional difference effect with the GCC48 / GCC49 / GCC5
    toolchains, but it breaks the build with CLANG9. Remove __inline__.
    
    Signed-off-by: Liming Gao <liming.gao@intel.com>
    Acked-by: Laszlo Ersek <lersek@redhat.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>