[llvm] [Offload] Make only a single thread handle the RPC server thread (PR #126067)

[Jon Chesterfield via llvm-commits]

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

Good fix. We might want a different strategy for host threads later but staying with one per device looks good for now.

https://github.com/llvm/llvm-project/pull/126067