sim: move register headers into sim/ namespace [PR sim/29869]

These headers define the register numbers for each port to implement
the sim_fetch_register & sim_store_register interfaces.  While gdb
uses these, the APIs are part of the sim, not gdb.  Move the headers
out of the gdb/ include namespace and into sim/ instead.

sim/m32c/gdb-if.c

@@ -33,7 +33,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 #include "sim/callback.h"
 #include "sim/sim.h"
 #include "gdb/signals.h"
-#include "gdb/sim-m32c.h"
+#include "sim/sim-m32c.h"
 
 #include "cpu.h"
 #include "mem.h"