rtld symlook_obj: move common code to check filtees into helper
rtld load_filtees(): reindent and reduce block nesting
rtld: ignore load_filtees() calls if we already loading filtees for the obj in addition to avoiding it for already loaded filtees. Issue is that during load, rtld needs to resolve some special ABI symbols, like executable stack fixer and static TLS initializer, which might trigger recursion. Example is libthr which is filter for libsys, and which exports __pthread_distribute_static_tls.