hurd: whitelist ld.so PLTs supposed to be avoided by rtld_hidden

* sysdeps/mach/hurd/localplt.data (ld.so): Add _dl_allocate_tls,
_dl_allocate_tls_init, _dl_exception_create, _dl_exception_create_format,
_dl_exception_free, _dl_find_dso_for_object, _dl_init_first, _dl_mcount,
___tls_get_addr, __tunable_get_val.