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/loader/elf_object.cc

@@ -141,7 +141,7 @@ ElfObject::ElfObject(ImageFileDataPtr ifd) : ObjectFile(ifd)
             "No loadable segments in '%s'. ELF file corrupted?\n",
             imageData->filename());
 
-    for (M5_VAR_USED auto &seg: image.segments())
+    for (GEM5_VAR_USED auto &seg: image.segments())
         DPRINTFR(Loader, "%s\n", seg);
 
     // We will actually read the sections when we need to load them