misc: Use compiler.hh macros when available.

Some places were hand coding __attribute__s when macros in compiler.hh
were available to do that job. Using the macros helps abstract away
compiler specific details and should be used when possible.

Change-Id: I94befebcfde2d673e874e9959588f69781bd9021
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35975
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/init.cc

@@ -51,6 +51,7 @@
 #include <string>
 #include <vector>
 
+#include "base/compiler.hh"
 #include "base/cprintf.hh"
 #include "base/logging.hh"
 #include "base/types.hh"
@@ -245,7 +246,7 @@ registerNativeModules()
  * Make the commands array weak so that they can be overridden (used
  * by unit tests to specify a different python main function.
  */
-const char * __attribute__((weak)) m5MainCommands[] = {
+M5_WEAK const char *m5MainCommands[] = {
     "import m5",
     "m5.main()",
     0 // sentinel is required