MdePkg: Do not use CreateEventEx unless required

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2446

There are many firmwares in the wild not supporting CreateEventEx,
including devices less than 5 years old.

Signed-off-by: Vitaly Cheptsov <vit9696@protonmail.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

MdePkg/Library/DxeRuntimeDebugLibSerialPort/DebugLib.c

@@ -77,9 +77,8 @@ DxeRuntimeDebugLibSerialPortConstructor (
     return Status;
   }
 
-  return SystemTable->BootServices->CreateEventEx (EVT_NOTIFY_SIGNAL,
+  return SystemTable->BootServices->CreateEvent (EVT_SIGNAL_EXIT_BOOT_SERVICES,
                                       TPL_NOTIFY, ExitBootServicesEvent, NULL,
-                                      &gEfiEventExitBootServicesGuid,
                                       &mEfiExitBootServicesEvent);
 }