UefiCpuPkg/CpuMpPei: Prepare for monitor buffer

UefiCpuPkg/CpuMpPei: Prepare for monitor buffer

Get AP loop mode to prepare for the monitor buffer required for ApInMwaitLoop
and ApInRunLoop.

Contributed-under: TianoCore Contribution Agreement 1.0
Cc: Feng Tian <feng.tian@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Tested-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19344 6f19259b-4bc3-4df7-8a09-765794883524