UefiCpuPkg/PiSmmCpuDxeSmm: patch "gSmmInitStack" with PatchInstructionX86()

Rename the variable to "gPatchSmmInitStack" so that its association with
PatchInstructionX86() is clear from the declaration, change its type to
X86_ASSEMBLY_PATCH_LABEL, and patch it with PatchInstructionX86(). This
lets us remove the binary (DB) encoding of some instructions in
"SmmInit.nasm".

The size of the patched source operand is (sizeof (UINTN)).

Cc: Eric Dong <eric.dong@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=866
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.c

@@ -848,7 +848,11 @@ PiCpuSmmEntry (
   //
   // Set SMI stack for SMM base relocation
   //
-  gSmmInitStack = (UINTN) (Stacks + mSmmStackSize - sizeof (UINTN));
+  PatchInstructionX86 (
+    gPatchSmmInitStack,
+    (UINTN) (Stacks + mSmmStackSize - sizeof (UINTN)),
+    sizeof (UINTN)
+    );
 
   //
   // Initialize IDT