JIT might accidentally use Apple LLVM 21 on macOS

[Fidget-Spinner]

Bug report

Bug description:

The JIT is incompatible with Apple LLVM 21, but sometimes it finds it and the tool check passes. We should block it.

Thanks to @kumaraditya303 for finding this issue.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-146475: Block Apple LLVM 21 from passing JIT tool version check #146478