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

[Michał Górny via llvm-commits]

mgorny wrote:

@jhuber6, any chance you could look at this? Not really sure this is the right solution, but it's consistent with OpenMP, and if it isn't, then we probably need to change OpenMP as well.

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