MdePkg: Use __builtin_offset with CLANGPDB toolchain

MdePkg: Use __builtin_offset with CLANGPDB toolchain

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2393

CLANGPDB does not define __GNUC__, but it does define __clang__. Check
for the __clang__ preprocessor definition to use __builtin_offsetof to
implement the OFFSET_OF macro.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Alex James <theracermaster@gmail.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>