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

[Nikita Popov via llvm-commits]

nikic wrote:

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

If you want to use CMAKE_INSTALL_LIBDIR, I think what you need to do is to use *only* CMAKE_INSTALL_LIBDIR, as we already include LLVM_LIBDIR_SUFFIX inside it. If we do that change, we should do it globally though: there are a lot more `lib${LLVM_LIBDIR_SUFFIX}` cases that you did not update here. We should also do it independently of other changes to rpath.

(Alternatively we could drop the code that adds LLVM_LIBDIR_SUFFIX to CMAKE_INSTALL_LIBDIR, and then use them together, but that seems less clean to me.)

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