UefiCpuPkg/PiSmmCpu: Fixed #double fault on #page fault.

This patch fixes https://bugzilla.tianocore.org/show_bug.cgi?id=246

Previously, when SMM exception happens after EndOfDxe,
with StackGuard enabled on IA32, the #double fault exception
is reported instead of #page fault.

Root cause is below:

Current EDKII SMM page protection will lock GDT.
If IA32 stack guard is enabled, the page fault handler will do task switch.
This task switch need write busy flag in GDT, and write TSS.

However, the GDT and TSS is locked at that time, so the
double fault happens.

We decide to not lock GDT for IA32 StackGuard enabled.

This issue does not exist on X64, or IA32 without StackGuard.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Jeff Fan <jeff.fan@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>