UefiCpuPkg/PiSmmCpuDxeSmm: Remove PSD layout assumptions

https://bugzilla.tianocore.org/show_bug.cgi?id=277

Remove dependency on layout of PROCESSOR_SMM_DESCRIPTOR
everywhere possible.  The only exception is the standard
SMI entry handler template that is included with the
PiSmmCpuDxeSmm module.  This allows an instance of the
SmmCpuFeaturesLib to provide alternate
PROCESSOR_SMM_DESCRIPTOR structure layouts.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jeff Fan <jeff.fan@intel.com>
Cc: Feng Tian <feng.tian@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>

UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/SmiEntry.nasm

@@ -22,6 +22,9 @@
 %define MSR_EFER      0xc0000080
 %define MSR_EFER_XD   0x800
 
+;
+; Constants relating to PROCESSOR_SMM_DESCRIPTOR
+;
 %define DSC_OFFSET 0xfb00
 %define DSC_GDTPTR 0x30
 %define DSC_GDTSIZ 0x38