[llvm] [openmp] [offload][openmp] - Remove standalone build in favor of 'runtimes' (PR #170693)

[Michał Górny via llvm-commits]

mgorny wrote:

How is it supposed to work? FWICS `llvm_gtest` target requires `find_package(LLVM)` to succeed but the runtimes CMakeLists has only:

```
find_package(LLVM PATHS "${LLVM_BINARY_DIR}" NO_DEFAULT_PATH NO_CMAKE_FIND_ROOT_PATH)
```

which cannot succeed because `LLVM_BINARY_DIR` is empty and `NO_DEFAULT_PATH` suppresses any way to specify the correct path.

https://github.com/llvm/llvm-project/pull/170693