MdePkg: Update Base.h to fix compilation issues with ICC.

MdePkg: Update Base.h to fix compilation issues with ICC.

Recent versions of the Intel C compiler define the _MSC_EXTENSIONS
constant. Base.h checks if this constant is defined to decide whether
or not to use a pragma intrinsic, which is unsupported by the latest
version of the Intel C compiler. Thus the check has been modified to
only pass in the case __INTEL_COMPILER is not defined.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Marvin Haeuser <Marvin.Haeuser@outlook.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>