[llvm] [Offload][WIP] Move `/openmp/libomptarget` to `/offload` (PR #82459)

[Eric Wright via llvm-commits]

efwright wrote:

@jplehr I'm helping @jdoerfert with this PR. I added a fix that should hopefully fix the build error you were seeing previously (I recreated it on my machines too). Let me know any other problems you encounter and I'll fix them as they come!

https://github.com/llvm/llvm-project/pull/82459