Add AT_* constants from Linux 6.12

[Florian Weimer]

* Nathan Chancellor:

> Is there going to be a patch submitted on the glibc side? I have been
> working around this locally in the kernel, should I submit this upstream
> since it is in a released glibc?

Most distributions that use glibc 2.42 will track the release branch.
We can fix that and backport it.  Someone just needs to write a patch. 8-)
(With just the fix, we can change the constant comparison code later.)