v0.0.0-7559-g54d29b76e

[XLS] Simplify apfloat using abs_diff

APFloat previously used a custom sign-magnitude difference function; this is now redundant with the new absolute difference library, so we switch to use that instead.

To enable this, we also add `abs_diff` to the list of built-in libraries, so it will be imported from the DSLX stdlib location.

PiperOrigin-RevId: 754159246