UefiCpuPkg/MpInitLib: Save CPU MP Data pointer

UefiCpuPkg/MpInitLib: Save CPU MP Data pointer

In PeiMpInitLib, save CPU MP Data pointer into one local Guided HOB.
In DxeMpInitLib, save CPU MP Data pointer into one global variable.

Add helper functions GetCpuMpData()/SaveCpuMpData().

v5:
  1. Move CPU_INIT_MP_LIB_HOB_GUID from MpLib.c to MpLib.h to make
     all C files visible.

Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Feng Tian <feng.tian@intel.com>
Cc: Giri P Mudusuru <giri.p.mudusuru@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Michael Kinney <michael.d.kinney@intel.com>