Offload architecture for portable targets

[trws]

In discussing updated sycl testing in RAJA, it came up that we currently don't have a way to specify what the target architectures are when enabling portable targets like SYCL and TARGET_OPENMP.

For sycl in dpcpp it looks like that would be -Xsycl-target-backend and possibly --offload-arch. For OpenMP offload it should be -fopenmp-targets=<targets>....

Just to make it extra fun, dpcpp supports sycl compiling to rocm, and rocm compiling to either hip or cuda. So... yeah, not sure about that one, but at least a way to list the targets intended for a given compilation would help. Would be nice if we could make it a single target list that will apply to whatever offload type is in use, maybe with per-type overrides if someone needs them?