gdb/testsuite/

gdb/testsuite/
	* gdb.threads/gcore-thread.exp (objfile, opts): New variables.
	Try to compile the test using -Wl,-z,norelro first.
	(load_core): New variable libthread_db_seen, initialize it.
	(zeroed-threads cannot be listed): Protect it by XFAIL on
	!$libthread_db_seen.