Fixed the issue in ExitBootServices() implementation to avoid the potential boot...

Fixed the issue in ExitBootServices() implementation to avoid the potential boot service usage after () after gCpu->DisableInterrupt ().

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gao, Liming <liming.gao@intel.com>
Reviewed-By: Olivier Martin <Olivier.martin@arm.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Zeng, Star <star.zeng@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15243 6f19259b-4bc3-4df7-8a09-765794883524