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

[Michał Górny via llvm-commits]

mgorny 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.

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