[Bug libstdc++/88466] [C++17] Support std::hardware_destructive_interference_size and std:: hardware_constructive_interference_size

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88466

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Duarte from comment #0)
> It seems, however, that neither std::hardware_destructive_interference_size
> nor std::hardware_constructive_interference_size have been included in
> libstdc++.

As clearly documented:
https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2017

The feature is broken by design and it's unclear how we want to proceed. Once
we pick a value we can never change it, because it becomes an ABI property.