Import xz 5.8.1

[emmatyping]

The current version of xz in source deps (5.2.5) is no longer supported https://tukaani.org/xz/#_development, so I figured I would update that here. Making this a draft for now while I make changes to the build system, e.g. python/cpython#140818.

Imported from https://github.com/tukaani-project/xz/releases/tag/v5.8.1

I also verified the gpg signature:

$ gpg --verify xz-5.8.1.tar.gz.sig xz-5.8.1.tar.gz
gpg: Signature made Thu 03 Apr 2025 04:38:23 AM PDT
gpg:                using RSA key 3690C240CE51B4670D30AD1C38EE757D69184620
gpg: Good signature from "Lasse Collin <lasse.collin@tukaani.org>" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 3690 C240 CE51 B467 0D30  AD1C 38EE 757D 6918 4620