UefiCpuPkg/DxeMpLib: Make sure APs in safe loop code

UefiCpuPkg/DxeMpLib: Make sure APs in safe loop code

Add one semaphore to make sure BSP to wait till all APs run in AP safe loop
code.

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>