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/cpu/pred/bpred_unit.cc

@@ -111,7 +111,7 @@ BPredUnit::drainSanityCheck() const
 {
     // We shouldn't have any outstanding requests when we resume from
     // a drained system.
-    for (M5_VAR_USED const auto& ph : predHist)
+    for (GEM5_VAR_USED const auto& ph : predHist)
         assert(ph.empty());
 }