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/base/stats/group.cc

@@ -67,7 +67,7 @@ Group::regStats()
 
     for (auto &g : statGroups) {
         if (Debug::Stats) {
-            M5_VAR_USED const SimObject *so =
+            GEM5_VAR_USED const SimObject *so =
                 dynamic_cast<const SimObject *>(this);
             DPRINTF(Stats, "%s: regStats in group %s\n",
                     so ? so->name() : "?",