MdeModulePkg/SmmLockBox: 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().

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Liming Gao <liming.gao@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/LockBox/SmmLockBox/SmmLockBox.c

@@ -77,10 +77,10 @@ SmmLockBoxSave (
     return ;
   }
   //
-  // The AsmLfence() call here is to ensure the above range check for the
-  // CommBuffer have been completed before calling into SaveLockBox().
+  // The SpeculationBarrier() call here is to ensure the above range check for
+  // the CommBuffer have been completed before calling into SaveLockBox().
   //
-  AsmLfence ();
+  SpeculationBarrier ();
 
   //
   // Save data
@@ -166,10 +166,10 @@ SmmLockBoxUpdate (
     return ;
   }
   //
-  // The AsmLfence() call here is to ensure the above range check for the
-  // CommBuffer have been completed before calling into UpdateLockBox().
+  // The SpeculationBarrier() call here is to ensure the above range check for
+  // the CommBuffer have been completed before calling into UpdateLockBox().
   //
-  AsmLfence ();
+  SpeculationBarrier ();
 
   //
   // Update data