UefiCpuPkg/MpLib.c: Add checking CR0 PG bit

If CR0 PG bit is not set, it means paging is not enabled on BSP. Thus, Execute
Disable feature is not working actually. Thus, we cannot enable it on APs.

v2:
  Correct the commit log.

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