add zvabd draft spec

[Qian Jiayan (keyon1992)]

reference:
https://github.com/riscv/integer-vector-absolute-difference/releases/tag/v0.6

[Qian Jiayan (keyon1992)]

Andrew Waterman (@aswaterman) hi, could you please help to review this PR

[Qian Jiayan (keyon1992)]

Renaming according to ARC review: https://lists.riscv.org/g/tech-unprivileged/topic/arc_minutes_2025_11_18/116467687

[Andrew Waterman (aswaterman)]

Please add the missing newline at the end of the file.

[Qian Jiayan (keyon1992)]

Please add the missing newline at the end of the file.

done

[Andrew Waterman (aswaterman)]

Qian Jiayan (@keyon1992) thank you!

Qian Jiayan (@keyon1992) / Ved Shanbhogue (@ved-rivos) do either of you recall if the ARC assigned these particular opcodes? They seem reasonable at a glance.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.08%. Comparing base (383cbca) to head (a82447a).
⚠️ Report is 45 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #381      +/-   ##
==========================================
+ Coverage   96.53%   97.08%   +0.55%     
==========================================
  Files          10       14       +4     
  Lines         750      926     +176     
==========================================
+ Hits          724      899     +175     
- Misses         26       27       +1     

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[Qian Jiayan (keyon1992)]

Qian Jiayan (@keyon1992) thank you!

Qian Jiayan (@keyon1992) / Ved Shanbhogue (@ved-rivos) do either of you recall if the ARC assigned these particular opcodes? They seem reasonable at a glance.

I think there are no vector fast tracks besides Sifve dot-products, so Zvabd/Zvzip opcodes should be reasonable.

[Qian Jiayan (keyon1992)]

Andrew Waterman (@aswaterman) Sorry, I accidentally closed the PR just now. I have reopened it, could you please help restart the CI.