UefiCpuPkg RegisterCpuFeaturesLib: Fix an ASSERTION issue

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=1968

We met assertion like below, it happens when there is only
one processor.

ASSERT_EFI_ERROR (Status = Not started)
ASSERT [CpuFeaturesDxe] X:\XXX\XXX\RegisterCpuFeaturesLib\
  DxeRegisterCpuFeaturesLib.c(149): !EFI_ERROR (Status)

The code should not call StartupAllAPs when there is only one processor.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Chandana Kumar <chandana.c.kumar@intel.com>
Cc: Kevin Li <kevin.y.li@intel.com>
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>

UefiCpuPkg/Library/RegisterCpuFeaturesLib/CpuFeaturesInitialize.c

@@ -1071,10 +1071,12 @@ CpuFeaturesDetect (
 
   CpuInitDataInitialize ();
 
+  if (CpuFeaturesData->NumberOfCpus > 1) {
     //
     // Wakeup all APs for data collection.
     //
     StartupAPsWorker (CollectProcessorData, NULL);
+  }
 
   //
   // Collect data on BSP

UefiCpuPkg/Library/RegisterCpuFeaturesLib/DxeRegisterCpuFeaturesLib.c

@@ -229,6 +229,7 @@ CpuFeaturesInitialize (
   OldBspNumber = GetProcessorIndex (CpuFeaturesData);
   CpuFeaturesData->BspNumber = OldBspNumber;
 
+  if (CpuFeaturesData->NumberOfCpus > 1) {
     Status = gBS->CreateEvent (
                     EVT_NOTIFY_WAIT,
                     TPL_CALLBACK,
@@ -242,11 +243,14 @@ CpuFeaturesInitialize (
     // Wakeup all APs for programming.
     //
     StartupAPsWorker (SetProcessorRegister, MpEvent);
+  }
+
   //
   // Programming BSP
   //
   SetProcessorRegister (CpuFeaturesData);
 
+  if (CpuFeaturesData->NumberOfCpus > 1) {
     //
     // Wait all processors to finish the task.
     //
@@ -254,6 +258,7 @@ CpuFeaturesInitialize (
       Status = gBS->CheckEvent (MpEvent);
     } while (Status == EFI_NOT_READY);
     ASSERT_EFI_ERROR (Status);
+  }
 
   //
   // Switch to new BSP if required

UefiCpuPkg/Library/RegisterCpuFeaturesLib/PeiRegisterCpuFeaturesLib.c

@@ -273,10 +273,13 @@ CpuFeaturesInitialize (
   // DXE type instance.
   //
 
+  if (CpuFeaturesData->NumberOfCpus > 1) {
     //
     // Wakeup all APs for programming.
     //
     StartupAPsWorker (SetProcessorRegister, NULL);
+  }
+
   //
   // Programming BSP
   //