misc: Replace M5_CLASS_VAR_USED with GEM5_CLASS_VAR_USED.

Change-Id: Ibd2230d684f41201b07fa9083881145e36176a68
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45241
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>

src/sim/probe/probe.hh

@@ -155,7 +155,7 @@ class ProbeManager
 {
   private:
     /** Required for sensible debug messages.*/
-    M5_CLASS_VAR_USED const SimObject *object;
+    GEM5_CLASS_VAR_USED const SimObject *object;
     /** Vector for name look-up. */
     std::vector<ProbePoint *> points;