ArmPkg: Fix Ecc error 8003

This patch fixes the following Ecc reported error:
The #ifndef at the start of an include file should have
one postfix underscore, and no prefix underscore character

Some include guards have been modified to match the name of the
header file. Some comments have also been added on the closing
'#endif'.

Cc: Bret Barkelew <bret.barkelew@microsoft.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Sami Mujawar <sami.mujawar@arm.com>
Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>

ArmPkg/Include/Library/DefaultExceptionHandlerLib.h

@@ -6,8 +6,8 @@
 
 **/
 
-#ifndef __DEFAULT_EXCEPTION_HANDLER_LIB_H__
-#define __DEFAULT_EXCEPTION_HANDLER_LIB_H__
+#ifndef DEFAULT_EXCEPTION_HANDLER_LIB_H_
+#define DEFAULT_EXCEPTION_HANDLER_LIB_H_
 
 /**
   This is the default action to take on an unexpected exception
@@ -22,4 +22,4 @@ DefaultExceptionHandler (
   IN OUT EFI_SYSTEM_CONTEXT           SystemContext
   );
 
-#endif
+#endif // DEFAULT_EXCEPTION_HANDLER_LIB_H_