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

[Nikita Popov via llvm-commits]

https://github.com/nikic requested changes to this pull request.

This change is incorrect. You can't combine CMAKE_INSTALL_LIBDIR and LLVM_LIBDIR_SUFFIX, that is going to result in `lib6464` libdirs.

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