[llvm] [openmp] [Offload] Remove standalone build in favor of 'runtimes' (PR #162904)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> I'll need to recheck but I'm pretty sure "standalone runtimes build" didn't work without the `OPENMP_STANDALONE_BUILD` var being set directly to disable some wrong assumptions.

I did a basic check and it seemed to build, but I didn't do extensive testing or anything. I'm wondering if we should be more explicit about depending on LLVM since it's possible the user would need to pass `LLVM_BINARY_DIR` manually since we depend on LLVM libraries..

https://github.com/llvm/llvm-project/pull/162904