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

[Johannes Doerfert via llvm-commits]

jdoerfert wrote:

> @jdoerfert I think you additionally need the cmake changes @jplehr posted a couple of days back. This one:
> 
> In current libomptarget we have this logic https://github.com/llvm/llvm-project/blob/fa2bbea14df3273b3403f34cc295c56233fdbd0d/openmp/libomptarget/CMakeLists.txt#L104C1-L120C8 but that is not present in the new CMake file.
> 
> Without these changes, I think you will see the OMPT tests as unsupported.

That logic was present the entire time in offload/CMakeLists.txt. 
I'll look into it.

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