MdeModulePkg/Variable: Update to consume SpeculationBarrier

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1417

Since BaseLib API AsmLfence() is a x86 arch specific API and should be
avoided using in generic codes, this commit replaces the usage of
AsmLfence() with arch-generic API SpeculationBarrier().

Please note that speculation execution barriers are intended to be
asserted for SMM codes, hence, this commit still preserve an empty
implementation of the speculation execution barrier for the DXE codes.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>

MdeModulePkg/Universal/Variable/RuntimeDxe/PrivilegePolymorphic.h

@@ -85,13 +85,15 @@ SetVariableCheckHandlerMor (
   );
 
 /**
-  This service is consumed by the variable modules to perform a serializing
-  operation on all load-from-memory instructions that were issued prior to the
-  call of this function.
+  This service is consumed by the variable modules to place a barrier to stop
+  speculative execution.
+
+  Ensures that no later instruction will execute speculatively, until all prior
+  instructions have completed.
 
 **/
 VOID
-MemoryLoadFence (
+VariableSpeculationBarrier (
   VOID
   );