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/Ia32/SmmInit.nasm

@@ -25,7 +25,7 @@ extern ASM_PFX(mSmmRelocationOriginalAddress)
 global ASM_PFX(gPatchSmmCr3)
 global ASM_PFX(gPatchSmmCr4)
 global ASM_PFX(gPatchSmmCr0)
-global ASM_PFX(gSmmInitStack)
+global ASM_PFX(gPatchSmmInitStack)
 global ASM_PFX(gcSmiInitGdtr)
 global ASM_PFX(gcSmmInitSize)
 global ASM_PFX(gcSmmInitTemplate)
@@ -72,8 +72,8 @@ BITS 32
     mov     fs, edi
     mov     gs, edi
     mov     ss, edi
-    DB      0xbc                        ; mov esp, imm32
-ASM_PFX(gSmmInitStack): DD 0
+    mov     esp, strict dword 0         ; source operand will be patched
+ASM_PFX(gPatchSmmInitStack):
     call    ASM_PFX(SmmInitHandler)
     rsm