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/cpu/o3/scoreboard.hh

@@ -58,7 +58,7 @@ class Scoreboard
     std::vector<bool> regScoreBoard;
 
     /** The number of actual physical registers */
-    M5_CLASS_VAR_USED unsigned numPhysRegs;
+    GEM5_CLASS_VAR_USED unsigned numPhysRegs;
 
   public:
     /** Constructs a scoreboard.