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).