[OpenMP] Adjust using the NVPTX architecture detection tool

A previous patch by @arsenm adjusted these to find the `amdgpu-arch`
tool correctly if we do a `LLVM_ENABLE_PROJECTS` build. This patch
applies the same to `nvptx-arch` tool to keep it consistent.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D154107