Switch to rust-ptx-linker

[termoshtt]

As described in termoshtt/accel#54

Use setting of rust-ptx-linker

{
    "arch": "nvptx64",
    "cpu": "sm_20",
    "data-layout": "e-i64:64-v16:16-v32:32-n16:32:64",
    "linker": "ptx-linker",
    "linker-flavor": "ld",
    "linker-is-gnu": true,
    "dll-prefix": "",
    "dll-suffix": ".ptx",
    "dynamic-linking": true,
    "llvm-target": "nvptx64-nvidia-cuda",
    "max-atomic-width": 0,
    "os": "cuda",
    "obj-is-bitcode": true,
    "panic-strategy": "abort",
    "target-endian": "little",
    "target-pointer-width": "64",
    "target-c-int-width": "32"
}

But we should use sm_60 because we assume Unified memory in Pascal.
https://docs.nvidia.com/cuda/pascal-compatibility-guide/index.html#building-pascal-compatible-apps-using-cuda-8-0

[termoshtt]

ptx-linker implements dead code elimination rust-accel/nvptx#11