UefiCpuPkg/DxeMpLib: Get safe AP loop handler from global variable

UefiCpuPkg/DxeMpLib: Get safe AP loop handler from global variable

AP loop function is already saved into global variable, needn't to get it from
AP function parameter.

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