CUDA C++ has the `__CUDA_ARCH__` macro for conditional compilation.
rust-cuda has a `CUDA_ARCH` environment variable that is similar, and
the `from_cuda_arch_env` method parses the environment variable's value
to produce a value of type `ComputeCapability`, which can be queried for
conditional compilation.

But `ComputeCapability` has a big problem. It's missing all the
capabilities after 80, including the 'a' and 'f' suffix ones. We could
just add them, but it implements `PartialOrd`/`Ord` and uses ordering to
determine feature availability. This was valid before the 'a' and 'f'
suffixes were added but is no longer, because some pairs of values are
incomparable. E.g. `100a` and `101a` -- each one has some features the
other doesn't, so neither is clearly larger than the other, and they're
also not equal.

So, what to do? Well, `CUDA_ARCH` was added in 2022. More recently,
another mechanism for conditional compilation was added:
`target_feature`, in Rust-GPU#239. This does work with the 'a' and 'f' suffix
targets, and it's more Rust-y.

So this commit just removes `CUDA_ARCH` and `ComputeCapability`
(removing two more places where the default compilation target is
specified) and changes the only uses (in `cuda_std/src/atomic/mid.rs`)
to use `target_feature` instead. We don't have any tests exercising
conditional compilation, alas, but I did some manual checking locally to
verify that it works the same.

CUDA C++ has the `__CUDA_ARCH__` macro for conditional compilation.
rust-cuda has a `CUDA_ARCH` environment variable that is similar, and
the `from_cuda_arch_env` method parses the environment variable's value
to produce a value of type `ComputeCapability`, which can be queried for
conditional compilation.

But `ComputeCapability` has a big problem. It's missing all the
capabilities after 80, including the 'a' and 'f' suffix ones. We could
just add them, but it implements `PartialOrd`/`Ord` and uses ordering to
determine feature availability. This was valid before the 'a' and 'f'
suffixes were added but is no longer, because some pairs of values are
incomparable. E.g. `100a` and `101a` -- each one has some features the
other doesn't, so neither is clearly larger than the other, and they're
also not equal.

So, what to do? Well, `CUDA_ARCH` was added in 2022. More recently,
another mechanism for conditional compilation was added:
`target_feature`, in Rust-GPU#239. This does work with the 'a' and 'f' suffix
targets, and it's more Rust-y.

So this commit just removes `CUDA_ARCH` and `ComputeCapability`
(removing two more places where the default compilation target is
specified) and changes the only uses (in `cuda_std/src/atomic/mid.rs`)
to use `target_feature` instead. We don't have any tests exercising
conditional compilation, alas, but I did some manual checking locally to
verify that it works the same.

CUDA C++ has the `__CUDA_ARCH__` macro for conditional compilation.
rust-cuda has a `CUDA_ARCH` environment variable that is similar, and
the `from_cuda_arch_env` method parses the environment variable's value
to produce a value of type `ComputeCapability`, which can be queried for
conditional compilation.

But `ComputeCapability` has a big problem. It's missing all the
capabilities after 80, including the 'a' and 'f' suffix ones. We could
just add them, but it implements `PartialOrd`/`Ord` and uses ordering to
determine feature availability. This was valid before the 'a' and 'f'
suffixes were added but is no longer, because some pairs of values are
incomparable. E.g. `100a` and `101a` -- each one has some features the
other doesn't, so neither is clearly larger than the other, and they're
also not equal.

So, what to do? Well, `CUDA_ARCH` was added in 2022. More recently,
another mechanism for conditional compilation was added:
`target_feature`, in Rust-GPU#239. This does work with the 'a' and 'f' suffix
targets, and it's more Rust-y.

So this commit just removes `CUDA_ARCH` and `ComputeCapability`
(removing two more places where the default compilation target is
specified) and changes the only uses (in `cuda_std/src/atomic/mid.rs`)
to use `target_feature` instead. We don't have any tests exercising
conditional compilation, alas, but I did some manual checking locally to
verify that it works the same.

CUDA C++ has the `__CUDA_ARCH__` macro for conditional compilation.
rust-cuda has a `CUDA_ARCH` environment variable that is similar, and
the `from_cuda_arch_env` method parses the environment variable's value
to produce a value of type `ComputeCapability`, which can be queried for
conditional compilation.

But `ComputeCapability` has a big problem. It's missing all the
capabilities after 80, including the 'a' and 'f' suffix ones. We could
just add them, but it implements `PartialOrd`/`Ord` and uses ordering to
determine feature availability. This was valid before the 'a' and 'f'
suffixes were added but is no longer, because some pairs of values are
incomparable. E.g. `100a` and `101a` -- each one has some features the
other doesn't, so neither is clearly larger than the other, and they're
also not equal.

So, what to do? Well, `CUDA_ARCH` was added in 2022. More recently,
another mechanism for conditional compilation was added:
`target_feature`, in #239. This does work with the 'a' and 'f' suffix
targets, and it's more Rust-y.

So this commit just removes `CUDA_ARCH` and `ComputeCapability`
(removing two more places where the default compilation target is
specified) and changes the only uses (in `cuda_std/src/atomic/mid.rs`)
to use `target_feature` instead. We don't have any tests exercising
conditional compilation, alas, but I did some manual checking locally to
verify that it works the same.