UefiCpuPkg/PiSmmCpu: Check XdSupport before set NX.

UefiCpuPkg/PiSmmCpu: Check XdSupport before set NX.

When XD is not supported, the BIT63 is reserved.
We should not set BIT63 in the page table.

Test OVMF IA32/IA32X64 with XD enabled/disabled.

Analyzed-by: Laszlo Ersek <lersek@redhat.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Jeff Fan <jeff.fan@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Laszlo Ersek <lersek@redhat.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>
Tested-by: Laszlo Ersek <lersek@redhat.com>