OpenMP::OpenMP_<lang> carries no device flags, and for GCC and Clang
links the OpenMP runtime without the OpenMP flag, so target regions
built through it fail to link or silently run on the host. Once
OpenMP_<lang>_OFFLOAD or OpenMP_<lang>_OFFLOAD_FLAGS is set, the new
targets add the device flags, plus the OpenMP flag at link time;
otherwise they match the host targets. FindOpenMP.Offload runs a
target kernel on a GPU and is enabled in the nvhpc job.
Fixes: #21784, #26701
