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/dev/net/sinic.cc

@@ -219,7 +219,7 @@ Device::read(PacketPtr pkt)
 
     prepareRead(cpu, index);
 
-    M5_VAR_USED uint64_t value = 0;
+    GEM5_VAR_USED uint64_t value = 0;
     if (pkt->getSize() == 4) {
         uint32_t reg = regData32(raddr);
         pkt->setLE(reg);

src/dev/net/tcp_iface.cc

@@ -251,7 +251,7 @@ TCPIface::connect()
 
 TCPIface::~TCPIface()
 {
-    M5_VAR_USED int ret;
+    GEM5_VAR_USED int ret;
 
     ret = close(sock);
     assert(ret == 0);