This adds the OMPT callbacks for the API functions disassociate_ptr and
associate_ptr.

Even if the device init callback is not registered, a tool should be
allowed to register other callbacks.

Many tests in the `offload` project have requirements defined by which
targets are not supported rather than which platforms are supported.
This patch aims to streamline the requirement definitions by adding four
new feature tags: `host`, `gpu`, `amdgpu`, and `nvidiagpu`.

In a nutshell, this moves our libomptarget code to populate the offload
subproject.

With this commit, users need to enable the new LLVM/Offload subproject
as a runtime in their cmake configuration.
No further changes are expected for downstream code.

Tests and other components still depend on OpenMP and have also not been
renamed. The results below are for a build in which OpenMP and Offload
are enabled runtimes. In addition to the pure `git mv`, we needed to
adjust some CMake files. Nothing is intended to change semantics.

```
ninja check-offload
```
Works with the X86 and AMDGPU offload tests

```
ninja check-openmp
```
Still works but doesn't build offload tests anymore.

```
ls install/lib
```
Shows all expected libraries, incl.
- `libomptarget.devicertl.a`
- `libomptarget-nvptx-sm_90.bc`
- `libomptarget.rtl.amdgpu.so` -> `libomptarget.rtl.amdgpu.so.18git`
- `libomptarget.so` -> `libomptarget.so.18git`

Fixes: https://github.com/llvm/llvm-project/issues/75124

---------

Co-authored-by: Saiyedul Islam <Saiyedul.Islam@amd.com>