[flang][cuda] Add missing interface for __powf (#153294)

`__powf` is defined in the CUDA Fortran programming guide but it's
missing from our cudadevice module. Add the interface and bind name to
`__nv_powf`


https://docs.nvidia.com/hpc-sdk/compilers/cuda-fortran-prog-guide/index.html#fortran-device-modules


https://docs.nvidia.com/cuda/libdevice-users-guide/__nv_powf.html#__nv_powf