UefiCpuPkg MpInitLib: Save/restore original WakeupBuffer for DxeMpLib

UefiCpuPkg MpInitLib: Save/restore original WakeupBuffer for DxeMpLib

Current code always allocates/frees < 1MB WakeupBuffer for DxeMpLib
until ExitBootService, but the allocation may be failed at late
phase of the boot.

This patch is to always save/restore original WakeupBuffer for
DxeMpLib, it is aligned with the solution for PeiMpLib at
9293d6e42e677e4a38e055258c0993ad8a9df14e, then AllocateResetVector()
and FreeResetVector() will be common and moved to MpLib.c.
Only difference is GetWakeupBuffer() that will be in PeiMpLib or
DxeMpLib respectively.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Jeff Fan <jeff.fan@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>