[libstdc++] Use __gthread_join in jthread/95989

[Florian Weimer]

* Alexandre Oliva via Gcc-patches:

> +    // Make sure it's not optimized out, not even with LTO.
> +    asm ("" : : "rm" (depend));

If the m constraint is used, this may never emit the symbol name and
thus not create a reference after all.