MdeModulePkg/CpuExceptionHanderLibNull: RegisterCpuInterruptHandler()

MdeModulePkg/CpuExceptionHanderLibNull: RegisterCpuInterruptHandler()

Current CpuExceptionHanderLibNull instance returns EFI_SUCCESS for all three
services. If platform does not want to hook the Exception vector for some
modules (For example DxeCore), it could select this NULL instance in DSC file
for those module. But some modules that want to consume
RegisterCpuInterruptHandler() cannot use NULL instance. If platform does not
select the correct library instance, it will does work. But the caller does not
recognize it.

This update is to return EFI_UNSUPPORTED on RegisterCpuInterruptHandler() in
NULL instance instead of return EFI_SUCCESS. Once platform selects this NULL
instance, the caller could know it from return status.

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: Michael Kinney <michael.d.kinney@intel.com>