[llvm] [offload] Permit redefining OPENMP_STANDALONE_BUILD (PR #157253)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 approved this pull request.

The handling here is weird because offload used to live in OpenMP, a lot of this needs to be cleaned up.

https://github.com/llvm/llvm-project/pull/157253