sim: create header namespace

The gdb/callback.h & gdb/remote-sim.h headers have nothing to do with
gdb and are really definitions for the libsim API under the sim/ tree.
While gdb uses those headers as a client, it's not specific to it.  So
create a new sim/ namespace and move the headers there.

sim/m32c/syscalls.c

@@ -25,7 +25,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 #include <unistd.h>
 #include <sys/time.h>
 
-#include "gdb/callback.h"
+#include "sim/callback.h"
 
 #include "cpu.h"
 #include "mem.h"