[libc] Ignore unknown CUDA versions for `libc` targeting NVPTX

Summary:
We generally need a CUDA toolchain to build the tests for the GPU `libc`
targeting NVPTX. However, clang will commonly emit warnings on versions
that are too new. We can ignore these in `libc` since we are manually
specifying the `+ptx` version to use whenever we compile. So we do not
need to worry about unexpected changes and we do not depend on any newer
features. So this should not be problematic.