sim: arm: move arch-specific settings to internal header

There's no need for these settings to be in sim-main.h which is shared
with common/ sim code, so move it all out to a new header which only
this port will include.

The BIT override would be better in the place where it's redefined, so
move it to armdefs.h instead.

sim/arm/armemu.h

@@ -14,6 +14,8 @@
     You should have received a copy of the GNU General Public License
     along with this program; if not, see <http://www.gnu.org/licenses/>. */
 
+#include "armdefs.h"
+
 extern ARMword isize;
 extern int trace;
 extern int disas;