OpenCL targets aren't aware of some math built-ins

I see the new function interface is much more careful about what's in the namespace (which is great!), but I think a number of the [built-in math functions](https://www.khronos.org/registry/OpenCL/specs/3.0-unified/html/OpenCL_C.html#math-functions) aren't currently registered. I tried to hack in, e.g., `round` to the lists of functions in `target/opencl.py` and `target/pyopencl.py` (e.g., [here](https://github.com/inducer/loopy/blob/main/loopy/target/pyopencl.py#L80) and [here](https://github.com/inducer/loopy/blob/main/loopy/target/pyopencl.py#L115)) but it still appeared to be unrecognized. If someone can point me in the right direction, I'd be happy to add the missing built ins.