MdeModulePkg: CpuExceptionHandlerLibNull: Return Success On Null Func

Currently, CpuExceptionHandlerLibNull returns EFI_UNSUPPORTED for
InitializeSeparateExceptionStacks. However, CpuMpPei, CpuDxe, and
DXE Core are all moving to call this function unconditionally and
expect it returns success. As such, the null lib is updated to
return success.

This fixes a hang on EmulatorPkg where DXE Core asserts if this
function returns an error.

Signed-off-by: Oliver Smith-Denny <osde@microsoft.com>

MdeModulePkg/Library/CpuExceptionHandlerLibNull/CpuExceptionHandlerLibNull.c

@@ -102,5 +102,5 @@ InitializeSeparateExceptionStacks (
   IN OUT UINTN  *BufferSize
   )
 {
-  return EFI_UNSUPPORTED;
+  return EFI_SUCCESS;
 }