sim: reorder header includes

We're including system headers after local headers in a bunch of
places, but this leads to conflicts when our local headers happen
to define symbols that show up in the system headers.

Use the more standard order of:
* config.h (via defs.h)
* system headers
* local library headers (e.g. bfd & libiberty)
* sim specific headers

sim/common/hw-alloc.c

@@ -20,11 +20,11 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 /* This must come before any other includes.  */
 #include "defs.h"
 
+#include <stdlib.h>
+
 #include "hw-main.h"
 #include "hw-base.h"
 
-#include <stdlib.h>
-
 struct hw_alloc_data
 {
   void *alloc;