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

Rename the variable to "gPatchSmiStack" so that its association with
PatchInstructionX86() is clear from the declaration. Also change its type
to X86_ASSEMBLY_PATCH_LABEL.

Unlike "gSmbase" in the previous patch, "gSmiStack"'s patched value is
also de-referenced by C code (in other words, it is read back after
patching): the InstallSmiHandler() function stores "CpuIndex" to the given
CPU's SMI stack through "gSmiStack". Introduce the local variable
"CpuSmiStack" in InstallSmiHandler() for calculating the stack location
separately, then use this variable for both patching into the assembly
code, and for storing "CpuIndex" through it.

It's assumed that "volatile" stood in the declaration of "gSmiStack"
because we used to read "gSmiStack" back for de-referencing; with that use
gone, we can remove "volatile" too. (Note that the *target* of the pointer
was never volatile-qualified.)

Finally, replace the binary (DB) encoding of "mov esp, imm32" 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>