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/python/m5/SimObject.py

@@ -766,7 +766,7 @@ class MetaSimObject(type):
         # method, or the Dummy one. Either an implementation is
         # mandantory since this was shunted off to the dummy class, or
         # one is optional which will override this weak version.
-        code('M5_VAR_USED ${{cls.cxx_class}} *')
+        code('GEM5_VAR_USED ${{cls.cxx_class}} *')
         code('Dummy${cls}Shunt<${{cls.cxx_class}}>::Params::create() const')
         code('{')
         code('    return Dummy${cls}Shunt<${{cls.cxx_class}}>::')