[PATCH] Add the __libc_single_threaded variable

[Florian Weimer]

* Florian Weimer:

> This version is based on the earlier feedback that a simple global
> variable should be used instead
>
>   <https://sourceware.org/ml/libc-alpha/2019-02/msg00073.html>
>
> and the benchmarking results suggesting that this is feasible:
>
>   <https://sourceware.org/ml/libc-alpha/2019-06/msg00954.html>
>
> The implementation is somewhat complicated by copy relocations, which
> are not applied to the inner namespace.
>
> This is probably the simplest possible implementation.  It does not
> attempt to reset the variable if there is only one thread left.

I'm going to post an implementation based on the early-init work for the
rseq addition.  This way, we can keep the variable in libc.so instead of
the loader, which is somewhat cleaner conceptually.

Thanks,
Florian