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/arm/wrapper.c

@@ -27,8 +27,8 @@
 #include <string.h>
 #include <bfd.h>
 #include <signal.h>
-#include "gdb/callback.h"
-#include "gdb/remote-sim.h"
+#include "sim/callback.h"
+#include "sim/sim.h"
 #include "sim-main.h"
 #include "sim-options.h"
 #include "armemu.h"