MdeModulePkg/Universal/StatusCodeHandler: Fix a bug about log lost

MdeModulePkg/Universal/StatusCodeHandler: Fix a bug about log lost

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3126

1. If use PeiDxeDebugLibReportStatusCode as DebugLib, then some logs
after ExitBootService() will be lost.
2. The root cause:
2.1 The original code will register an unregister function
of gEfiEventExitBootServicesGuid, this unregister function will call
EFI_RSC_HANDLER_PROTOCOL->Unregister and does not support log through
serial port.
2.2 And some other drivers also register call back funtions of
gEfiEventExitBootServicesGuid.
2.3 Then after the unregister function is called, other call back
functions can't out log if them use RSC as DebugLib.
3. The DxeMain will report status code EFI_SW_BS_PC_EXIT_BOOT_SERVICES
after notify all the call back functions of
gEfiEventExitBootServicesGuid.
4. Solution: the StatusCodeHandlerRuntimeDxe.c will not register an
unregister function of gEfiEventExitBootServicesGuid, but unregister it
after receive the status code of EFI_SW_BS_PC_EXIT_BOOT_SERVICES.

Cc: Dandan Bi <dandan.bi@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Signed-off-by: Ming Tan <ming.tan@intel.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Dandan Bi <dandan.bi@intel.com>