std::call_once without TLS

[Paolo Carlini]

Jonathan Wakely wrote:
> I proposed a fix as soon as I found the problem, three weeks before
> 4.4.0 was released.  I personally don't consider it important, as I
> don't use any platforms without TLS.
>   
Ok, I understand. I saw something around, in an interim form, was hoping
people more knowledgeable than me in this area could provide you some
feedback. Anyway, for the future, if you have changes which impact the
exported symbols very close to the release, make sure to ping it
properly and point out its urgency.
> I'm working on PRs 35569 and 38889 again, so I'll leave call_once
> alone now that it's being exported.
>   
Thanks a lot. I added Chris in CC for the PR. In case, please, keep in
contact. If I understand correctly, in case of no TLS of course we
cannot do any "miracle", but we must minimally make sure that user code
compiled with 4.4.0 still links with the 4.4.1 library (thus 3.4.11 vs
3.4.12).

Paolo.