Use the libthread_db 'td_symbol_list' function to look up all the

Use the libthread_db 'td_symbol_list' function to look up all the
symbols libthread_db will need when invited to by GDB.
* thread-db.c (add_symbol_to_list): Make 'name' argument const.
(td_symbol_list_p): New libthread_db function pointer.
(thread_db_dlopen): Initialize it.
(thread_db_attach): Use td_symbol_list to pre-load our symbol
cache with the names of all the symbols libthread_db will ever
request.