[llvm] [llvm] add gnu install dirs (PR #125376)

[Nikita Popov via llvm-commits]

nikic wrote:

> This was reverted in [e941b03](https://github.com/llvm/llvm-project/commit/e941b031d316f74267350fe2373d30e28b3a2c2e) by @Ericson2314.

What has changed in your variant of the patch to address the failures that resulted in the previous revert?

> The goal here is to properly get this patch into LLVM upstream and drop the patch from nixpkgs. This decreases the maintenance burden as we'd no longer need to rely on maintaining this patch.

It would be more helpful if the PR description explained what issue this patch is actually trying to solve. Presumably nixpkgs carries this patch for a reason :)

https://github.com/llvm/llvm-project/pull/125376