UefiCpuPkg/MpInitLib: Move allocating reserved memory for AP loop code

UefiCpuPkg/MpInitLib: Move allocating reserved memory for AP loop code

In Exit Boot Services callback function, we cannot use allocate memory services
because it may change the memory map that has been gotten by OS.

This fix is to move allocating reserved memory for AP loop code to
InitMpGlobalData() and save the memory address in one global variable.

Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Feng Tian <feng.tian@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>