[ihb2032]

What and why

• Document why the RVV gate requires GCC >= 14 / Clang >= 19 (first releases with the stable RVV 1.0 intrinsic API).
• Fix the preprocessor nesting of the version check and drop a stray # between the arch blocks.

Checklist

• I searched open pull requests and issues: this problem is not already being addressed.
• Bug fix: it comes with a reproducer or a test that fails without the fix.
• Performance change: numbers from repeated runs are included, with CPU, compiler, flags and data.
  A gain in one configuration is not a general gain: see the Performance section of CONTRIBUTING.md.
• I stated below whether AI or other automated tools were used.

Tools used

None