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

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

"mXdSupported" is a global BOOLEAN variable, initialized to TRUE. The
CheckFeatureSupported() function is executed on all processors (not
concurrently though), called from SmmInitHandler(). If XD support is found
to be missing on any CPU, then "mXdSupported" is set to FALSE, and further
processors omit the check. Afterwards, "mXdSupported" is read by several
assembly and C code locations.

The tricky part is *where* "mXdSupported" is allocated (defined):

- Before commit 717fb60443fb ("UefiCpuPkg/PiSmmCpuDxeSmm: Add paging
  protection.", 2016-11-17), it used to be a normal global variable,
  defined (allocated) in "SmmProfile.c".

- With said commit, we moved the definition (allocation) of "mXdSupported"
  into "SmiEntry.nasm". The variable was defined over the last byte of a
  "mov al, 1" instruction, so that setting it to FALSE in
  CheckFeatureSupported() would patch the instruction to "mov al, 0". The
  subsequent conditional jump would change behavior, plus all further read
  references to "mXdSupported" (in C and assembly code) would read back
  the source (imm8) operand of the patched MOV instruction as data.

  This trick required that the MOV instruction be encoded with DB.

In order to get rid of the DB, we have to split both roles: we need a
label for the code patching, and "mXdSupported" has to be defined
(allocated) independently of the code patching. Of course, their values
must always remain in sync.

(1) Reinstate the "mXdSupported" definition and initialization in
    "SmmProfile.c" from before commit 717fb60443fb. Change the assembly
    language definition ("global") to a declaration ("extern").

(2) Define the "gPatchXdSupported" label (type X86_ASSEMBLY_PATCH_LABEL)
    in "SmiEntry.nasm", and add the C-language declaration to
    "SmmProfileInternal.h". Replace the DB with the MOV mnemonic (keeping
    the imm8 source operand with value 1).

(3) In CheckFeatureSupported(), whenever "mXdSupported" is set to FALSE,
    patch the assembly code in sync, with PatchInstructionX86().

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>