[llvm] [NVPTX] pass correct GPU arch to ptxas test (PR #81535)

[Artem Belevich via llvm-commits]

Artem-B wrote:

> Seems I might not have these tests run locally despite having CUDA installed. Anything special you need to enable it?

Just enable the optional ptxas tests by pointing LLVM_PTXAS_EXECUTABLE to the ptxas.
```
LLVM_PTXAS_EXECUTABLE=$HOME/local/cuda-11.8.0/bin/ptxas ninja check-llvm
```

https://github.com/llvm/llvm-project/pull/81535