UefiCpuPkg/DxeMpInitLib: Support source debugging on AP function

UefiCpuPkg/DxeMpInitLib: Support source debugging on AP function

The current DxeDebugAgentLib supports source debugging on AP function. This
update is to update DxeMpInitLib to consume Debug Agent Library by
DEBUG_AGENT_INIT_DXE_AP flag. Thus, we could source debugging AP function
invoked by CPU MP Protocol.

However, current SecPeiDebugAgentLib does not support source debugging on AP
function invoked by CPU MP PPI. I have submitted one bugzilla to add this
support at https://bugzilla.tianocore.org/show_bug.cgi?id=308.

Cc: Feng Tian <feng.tian@intel.com>
Cc: Kinney Michael D <michael.d.kinney@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@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>