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

[Michał Górny via llvm-commits]

mgorny wrote:

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

Yeah, I more meant about having to redefine it manually when building via `runtimes/` but I guess we can look into a more permanent, cross-project solution later.

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