It can be used in `NvvmOption::from_str`, and will also be used in a
subsequent commit in compiletests.

It's possible with the `#[default]` attribute.

…mpute_75`.

CUDA 12.0 was released in December 2022, and CUDA 13.0 was released in
August 2025. It feels like a good time to drop CUDA 11.x support. This
means later Kepler devices with compute capabilities of 3.5 and 3.7 will
no longer be supported. The minimum version of NVVM IR increases from
1.6 to 2.0, because that's what CUDA 12.0 uses.

Along with this, change the default compiler target to `compute_75`.
This is a good choice because it's the minimum supported by CUDA 13.0,
and gets Rust CUDA a step closer to working with CUDA 13.0.

The existing defaults were all over the place.
- `NvvmArch::default()` was `compute_52`.
- `CudaBuilder`'s default was `compute_61`.
- compiletest's default was `compute_70`.

This commit makes the latter two determined by `NvvmArch::default()`,
which is changed to `compute_75`.

Currently CI runs compiletests on `compute_61`, `compute_70`, and
`compute_90`; this commit changes the `compute_70` to `compute_75`. It
seems sensible to have the default value as one of the things tested by
CI.

This comment also adds a comment on NvvmArch with a table of
CUDA/`compute_*` values, which I found very useful.

Resources:
- https://en.wikipedia.org/wiki/CUDA#GPUs_supported for compute
  capabilities supported by different CUDA versions.
- https://docs.nvidia.com/cuda/archive/12.0.0/cuda-toolkit-release-notes/index.html
  for NVVM IR version information.

This got dropped in a previous commit and is making CI fail with:
```
--- stderr

  thread 'main' panicked at /root/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/bindgen-0.71.1/lib.rs:604:27:
  Unable to find libclang: "couldn't find any valid shared libraries matching: ['libclang.so', 'libclang-*.so', 'libclang.so.*', 'libclang-*.so.*'], set the `LIBCLANG_PATH` environment variable to a path where one of these files can be found (invalid: [])"
  stack backtrace:
     0: __rustc::rust_begin_unwind
               at /rustc/f34ba774c78ea32b7c40598b8ad23e75cdac42a6/library/std/src/panicking.rs:697:5
     1: core::panicking::panic_fmt
               at /rustc/f34ba774c78ea32b7c40598b8ad23e75cdac42a6/library/core/src/panicking.rs:75:14
     2: core::result::unwrap_failed
               at /rustc/f34ba774c78ea32b7c40598b8ad23e75cdac42a6/library/core/src/result.rs:1852:5
     3: core::result::Result<T,E>::expect
     4: bindgen::ensure_libclang_is_loaded::{{closure}}
     5: std::sync::once_lock::OnceLock<T>::get_or_init::{{closure}}
     6: std::sync::once_lock::OnceLock<T>::initialize::{{closure}}
     7: std::sync::poison::once::Once::call_once_force::{{closure}}
     8: std::sys::sync::once::futex::Once::call
               at /rustc/f34ba774c78ea32b7c40598b8ad23e75cdac42a6/library/std/src/sys/sync/once/futex.rs:178:21
     9: std::sync::poison::once::Once::call_once_force
    10: std::sync::once_lock::OnceLock<T>::initialize
    11: std::sync::once_lock::OnceLock<T>::get_or_try_init
    12: std::sync::once_lock::OnceLock<T>::get_or_init
    13: bindgen::ensure_libclang_is_loaded
    14: bindgen::Bindings::generate
    15: bindgen::Builder::generate
    16: build_script_main::create_cuda_driver_bindings
    17: build_script_main::main
    18: core::ops::function::FnOnce::call_once
  note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
```