[RFC] Add stdckdint.h header for C23

[Florian Weimer]

* Paul Eggert:

> I don't see how you could implement __has_include_next(<stdckdint.h>)
> for arbitrary non-GCC compilers, which is what we'd need for glibc
> users.

This is not a requirement for glibc in general.  For example, <math.h>
only works with compilers to which it has been ported.

Thanks,
Florian