Stop gdb in gnat_runtime_has_debug_info

gnat_runtime_has_debug_info starts a new gdb to do its work.  However,
it also leaves this gdb running, which can potentially confuse the
calling test -- I encountered this when writing a new DAP test.  This
patch changes the proc to shut down gdb.

gdb/testsuite/lib/ada.exp

@@ -204,5 +204,7 @@ gdb_caching_proc gnat_runtime_has_debug_info {} {
 	}
     }
 
+    gdb_exit
+
     return $has_debug_info
 }