rustc-book: update sys-v abi link

[vilgotf]

• I did not use an LLM to create a change in this PR.
• I used an LLM to create a change in this PR, and I have explained below how it was used.

Upstream has moved to a new location and the current link is 6 years out of date. See https://github.com/hjl-tools/x86-psABI/

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