[patch] libstdc++/65473 Make <ciso646> define libstdc++ version macros.

[Jonathan Wakely]

This change would allow including <ciso646> to be used to check for
__GLIBCXX__ and detect whether youre using libstdc++ or not. Howard
Hinnant recommends including that header for libc++ because it has no
other effects in C++.

We could make every <cxxx> header include <bits/c++config.h> so that
any of them can be used, but I can't be bothered doing that change!
This makes it work for the one header that is recommended to be used,
but of course that doesn't help people using older versions of
libstdc++, who still need to include some other header.

Is this worth doing?

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