UefiCpuPkg/PiSmmCpuDxeSmm: Add a new mIsShadowStack flag

UefiCpuPkg/PiSmmCpuDxeSmm: Add a new mIsShadowStack flag

This patch is code refactoring and doesn't change any functionality.
Add a new mIsShadowStack flag to identify whether current memory is
shadow stack. Previous smm code logic regards a RO range as shadow
stack and set the dirty bit in corresponding page table entry if
mInternalCr3 is not 0, which may be confusing.

Signed-off-by: Dun Tan <dun.tan@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>