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/testers/memtest/memtest.cc

@@ -243,7 +243,7 @@ MemTest::tick()
     if (cmd < percentReads) {
         // start by ensuring there is a reference value if we have not
         // seen this address before
-        M5_VAR_USED uint8_t ref_data = 0;
+        GEM5_VAR_USED uint8_t ref_data = 0;
         auto ref = referenceData.find(req->getPaddr());
         if (ref == referenceData.end()) {
             referenceData[req->getPaddr()] = 0;