math: Sync atanh from CORE-MATH

[Paul Zimmermann]

about https://patchwork.sourceware.org/project/glibc/patch/20260106132005.2570514-1-adhemerval.zanella@linaro.org/

I did review that patch and it is ok for me.

The changes in polydd() do simplify some code that implements fasttwosum().

The change in as_atanh_zero() just permutes the operands of the muldd()
call. Looking at the code of muldd, this does not change the result
since addition is commutative (alhh + ahlh = ahlh + alhh).

The other changes add comments or new non-regression tests.

I have to say however that I am one of the authors of CORE-MATH.
Another eye would be welcome if needed.

Reviewed-by: Paul Zimmermann <Paul.Zimmermann@inria.fr>

Paul