PR corefile/8210

	PR corefile/8210

	* linux-thread-db.c (add_thread_db_info): Skip glibc/BZ5983
	workaround on core files.
	(try_thread_db_load_1): Don't try enabling thread event reporting
	on core files.
	(thread_db_load): Allow thread_db on core files.
	(attach_thread): Don't check thread signals on core files, nor try
	really attaching to the thread, nor enabling thread event event
	reporting.
	(thread_db_detach): Don't try disabing thread event reporting or
	removing thread event breakpoints when debugging a core file.
	(find_new_threads_callback): Don't try enabling thread event
	reporting on core files.
	(thread_db_find_new_threads_2): Don't look for a stopped lwp when
	debugging a core file.
	(thread_db_find_new_threads): Don't update thread
	cores (processors) when debugging a core (dump).