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

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>