[Libomptarget] Replace Nvidia arch lookup with 'nvptx-arch'

This method to look up the CUDA architecture is deprecated in newer
versions of CMake. We also have our own way to query this information
that we control now via the `nvptx-arch` program, which should always be
present in LLVM builds with clang going forward. This is currently only
used for testing so I think we should be okay with the dependency.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D141933