UefiCpuPkg/PiSmmCpuDxeSmm: 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>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>

UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.c

@@ -238,10 +238,10 @@ SmmReadSaveState (
     return EFI_INVALID_PARAMETER;
   }
   //
-  // The AsmLfence() call here is to ensure the above check for the CpuIndex
-  // has been completed before the execution of subsequent codes.
+  // The SpeculationBarrier() call here is to ensure the above check for the
+  // CpuIndex has been completed before the execution of subsequent codes.
   //
-  AsmLfence ();
+  SpeculationBarrier ();
 
   //
   // Check for special EFI_SMM_SAVE_STATE_REGISTER_PROCESSOR_ID