gh-138497: Support LLVM_VERSION configuration via env

[danigm]

This patch makes possible to override the llvm version used during build using "LLVM_VERSION" and "EXTERNALS_LLVM_TAG" environment variables.

This is useful to test the build with other LLVM versions without changing the sources, for example to build with llvm-20.

Related issue: #136895

• Issue: Make LLVM_VERSION configurable with env variable #138497

[brandtbucher]

If we do this:

• We should do it in a manner similar to CFLAGS_JIT, where the env var is captured by the configure script and passed via the command-line to the build script.
• EXTERNALS_LLVM_TAG should be left alone, I don't think it makes sense for anyone to override that (it's only used for Windows builds and it's literally a tag in a repo we control).

[danigm]

If we do this:

* We should do it in a manner similar to `CFLAGS_JIT`, where the env var is captured by the `configure` script and passed via the command-line to the build script. * `EXTERNALS_LLVM_TAG` should be left alone, I don't think it makes sense for anyone to override that (it's only used for Windows builds and it's literally a tag in a repo we control). 

I've done that way. Get the env variable in configure and pass to Tools/jit/build.py script as a new argument --llvm-version.

[brandtbucher]

@savannahostrowski, thoughts on this change?

[savannahostrowski]

Thanks for this! I think this change is fair, so long as we only advertise that a single LLVM version is officially supported.

[bedevere-app]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[danigm]

I have made the requested changes; please review again

[bedevere-app]

Thanks for making the requested changes!

@savannahostrowski: please review the changes made to this pull request.

[danigm]

I have made the requested changes; please review again

[bedevere-app]

Thanks for making the requested changes!

@savannahostrowski: please review the changes made to this pull request.

[savannahostrowski]

Thanks for making the updates - I have a couple of other small wording suggestions but I think otherwise this looks good.

Would you also mind updating the Tools/jit/README.md to reflect this change? Right now, the README says that LLVM 19 is required. With this change, we should call out that LLVM 19 is the officially supported version, but that you can modify it if need be, using the env var.