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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

The offloading runtime is unique because it relies on the LLVM binary directory, whereas most others can be built standalone. There's already a case where we do `find_package(LLVM)` in the runtimes handling but it's not a requirement. We could probably add some more useful diagnostic there, like if someone's enabled offloading and we can't find LLVM we suggest setting that and fatal error.

Another issue with the offloading build is that we rely on a tablegen tool for the liboffload part, which means we can't do real cross-compiling like runtimes implies. This should probably be moved to LLVM and just built by default. It should hopefully show up in the NATIVE directory and we can fish it out from there.

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