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

Rename the variable to "gPatchSmbase" 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
"SmiEntry.nasm".

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/X64/SmiEntry.nasm

@@ -53,7 +53,7 @@ extern ASM_PFX(gSmiHandlerIdtr)
 extern ASM_PFX(CpuSmmDebugEntry)
 extern ASM_PFX(CpuSmmDebugExit)
 
-global ASM_PFX(gSmbase)
+global ASM_PFX(gPatchSmbase)
 global ASM_PFX(mXdSupported)
 global ASM_PFX(gSmiStack)
 global ASM_PFX(gSmiCr3)
@@ -75,8 +75,8 @@ _SmiEntryPoint:
 o32 lgdt    [cs:bx]                       ; lgdt fword ptr cs:[bx]
     mov     ax, PROTECT_MODE_CS
     mov     [cs:bx-0x2],ax
-    DB      0x66, 0xbf                   ; mov edi, SMBASE
-ASM_PFX(gSmbase): DD 0
+    mov     edi, strict dword 0           ; source operand will be patched
+ASM_PFX(gPatchSmbase):
     lea     eax, [edi + (@ProtectedMode - _SmiEntryPoint) + 0x8000]
     mov     [cs:bx-0x6],eax
     mov     ebx, cr0