UefiCpuPkg/MpInitLib: Send INIT-SIPI-SIPI to get processor count

UefiCpuPkg/MpInitLib: Send INIT-SIPI-SIPI to get processor count

CollectProcessorCount() will send the 1st INIT-SIPI-SIPI to get processor count
in system.

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>