Allow registry to refer to const types

So far, the registry hasn't been used to refer to a 'const' type, but
this changes with the gdbarch change.  This patch arranges to let the
registry store a pointer-to-const, by removing const in the 'set'
method.

gdb/registry.h

@@ -117,7 +117,7 @@ public:
     void set (T *obj, DATA *data) const
     {
       registry<T> *reg_obj = registry_accessor<T>::get (obj);
-      reg_obj->set (m_key, data);
+      reg_obj->set (m_key, (typename std::remove_const<DATA> *) data);
     }
 
     /* If this key uses the default deleter, then this method is