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

[Jan Patrick Lehr via llvm-commits]

jplehr wrote:

I have checked out this PR and tried our current buildbot config in a fresh build directory.
```
cmake -G Ninja ../llvm-project/llvm -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON '-DLLVM_LIT_ARGS=-vv --show-unsupported --timeout --show-xfail -j 64' -DCMAKE_INSTALL_PREFIX=/tmp/llvm.inst -DCMAKE_BUILD_TYPE=Release -DLLVM_USE_LINKER=lld '-DLLVM_TARGETS_TO_BUILD=X86;AMDGPU' '-DLLVM_ENABLE_RUNTIMES=openmp' '-DLLVM_ENABLE_PROJECTS=clang;lld;llvm;compiler-rt' -DCLANG_DEFAULT_LINKER=lld
```
as expected, this enables the OpenMP project and OpenMP tests.

I have then nuked the build directory, kept everything in the CMake call the same, but added `offload` as second `'-DLLVM_ENABLE_RUNTIMES=openmp;offload'`. However, that did not lead to `check-openmp` run offloading tests.
I have tried `check-libomptarget` which gave me the error: `ninja: error: unknown target 'check-libomptarget'`. I have tried `ninja -C runtimes/runtimes-bins check-libomptarget` which similarly resulted in an error
```
ninja: Entering directory `runtimes/runtimes-bins'
ninja: error: unknown target 'check-libomptarget'
```

What would I need to change to enable offloading tests?

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