main/llvm18: fix soname in -libs subpackage

Maybe some further refinements are needed, and if i am not around later today, anyone is welcome to continue building on what i have done here.

The changes here allow Rust 1.78 to be rebuilt with LLVM18 (though there are other issues with Rust itself to look into after 3.20 is out), and trying to rebuild Rust was how i stumbled upon this issue with llvm18-libs in the first place.

For context, as i mentioned on IRC, while being built, Rust 1.78 tries to run /usr/lib/llvm18/bin/llvm-config --link-shared --libs which fails with error: libLLVM-18.so is missing. After applying the changes here, i no longer see that error while building Rust.