UefiCpuPkg/PiSmmCpuDxeSmm: Enable/Restore XD in SMM

If XD is supported, then SMM enables it.  The non-SMM execution
environment can choose to enable or disable XD, so the state of
XD must be detected in each SMI and be enabled/restored.

Cc: Jeff Fan <jeff.fan@intel.com>
Cc: Jiewen Yao <jiewen.yao@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>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>