misc: Replace M5_VAR_USED with GEM5_VAR_USED.

Change-Id: I64a874ccd1a9ac0541dfa01971d7d620a98c9d32
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45231
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>

src/arch/isa_parser/isa_parser.py

@@ -133,7 +133,7 @@ class Template(object):
             if operands.predRead:
                 myDict['op_decl'] += 'uint8_t _sourceIndex = 0;\n'
             if operands.predWrite:
-                myDict['op_decl'] += 'M5_VAR_USED uint8_t _destIndex = 0;\n'
+                myDict['op_decl'] += 'GEM5_VAR_USED uint8_t _destIndex = 0;\n'
 
             is_src = lambda op: op.is_src
             is_dest = lambda op: op.is_dest