[libclc] [llvm] [flang-rt][libclc] Install multilib variants in their own directory (PR #230251)

[Wenju He via llvm-commits]

https://github.com/wenju-he approved this pull request.

libclc change LGTM.
Is there a libclc multilib use case, or is this for consistency?

https://github.com/llvm/llvm-project/pull/230251