[llvm] [openmp] [Offload] Move `/openmp/libomptarget` to `/offload` (PR #75125)

[Ye Luo via llvm-commits]

ye-luo wrote:

libomptarget files install location is changed back to `<install>/lib` from `<install>/lib/x86_64-unknown-linux-gnu`

https://github.com/llvm/llvm-project/pull/75125