[OpenMP] Make Xopenmp-target args compile-only to silence warnings

Summary:
Previously we needed the `Xopenmp-target=` option during the linking
phase so the old offloading driver knew which items to extract and link
for the device. Now that the new driver has become the default this is
no longer necessary and will cause a warning to be emitted for the
unused argument. This should be silenced to avoid noise.