libdebugger: ARM fixes for Cortex-A8 and ARM mode.

- Fix destorying the target and thread parts.
- Fix the ARM backend to support Cortex-A8 and ARM mode code.
- Use the DBGDSCR interrupt mask when single stepping.
- Use the DBGDSCR method of entry to debug mode to filter the
  execptions.
- Add support for BSPs to control the ARM backend.

cpukit/libdebugger/rtems-debugger-server.c

@@ -1704,13 +1704,13 @@ rtems_debugger_session(void)
 
   r = rtems_debugger_target_create();
   if (r < 0) {
-    rtems_debugger_thread_destroy();
     rtems_debugger_unlock();
     return r;
   }
 
   r = rtems_debugger_thread_create();
   if (r < 0) {
+    rtems_debugger_target_destroy();
     rtems_debugger_unlock();
     return r;
   }
@@ -1725,8 +1725,8 @@ rtems_debugger_session(void)
                                  0,
                                  &rtems_debugger->events_task);
   if (r < 0) {
-    rtems_debugger_target_destroy();
     rtems_debugger_thread_destroy();
+    rtems_debugger_target_destroy();
     rtems_debugger_unlock();
     return r;
   }