MdePkg BaseIoLibIntrinsic: Remove __inline__ attribute for IO functions

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>