[llvm] [openmp] [OpenMP][Offload] Support standalone build (PR #88957)

[Johannes Doerfert via llvm-commits]

jdoerfert wrote:

> Looks better now, if it works for ROCm's purposes then let's ship it.

@saiislam @estewart08 Does this work for ROCm? Can I pick it up?

https://github.com/llvm/llvm-project/pull/88957