MdePkg: Add PcdSpeculationBarrierType

MdePkg: Add PcdSpeculationBarrierType

Add gEfiMdePkgTokenSpaceGuid.PcdSpeculationBarrierType that
uses the PCD type FixedAtBuild.  This performs a build time
selection for the type of speculation barrier to use in the
BaseLib function SpeculationBarrier().  The recommended
speculation barrier for x86 is LFENCE and this is the default
value for this PCD.  x86 CPUs that do not support LFENCE must
select one of the other supported values which includes CPUID
and nothing.

Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Brian J. Johnson <brian.johnson@hpe.com>