gdb/

gdb/
	* linux-thread-db.c (find_new_threads_callback): Exit on zero TI_TID
	even if !TARGET_HAS_EXECUTION.

gdb/testsuite/
	* gdb.threads/gcore-thread.exp ($core0file): New variable.
	(clear __stack_user.next, clear stack_used.next)
	(save a zeroed-threads corefile): New test.
	Call core_load for $core0file.
	(zeroed-threads cannot be listed): New test.