Unified SYCL+CUDA builds

[intelmatt]

I needed xgboost builds to run on both SYCL & CUDA, so implemented a prototype here: https://github.com/intelmatt/xgboost/tree/sycluda

I don't plan to upstream to xgboost since the above meets my personal needs, but if you see anything valuable there, let me know and I can clean it up and submit PRs.

CC @razdoburdin @avolkov-intel @RAMitchell