Detect CPU features at build time

[ogxd]

No description provided.

[notsatvrn]

is_aarch64_feature_detected is available, and also is_arm_feature_detected with the stdsimd feature enabled. ARM32 support actually depends on this feature to work, something I didn't notice when writing #53. These should be added.

[ogxd]

I tried using https://github.com/cross-rs/cross to try out building for various platforms. It seems the target_arch in the build.rs does not match the actual target platform for some reason, while the environment variable CARGO_CFG_TARGET_ARCH does have the appropriate value. This is annoying because using is_x86_feature_detected requires usage #[cfg(any(target_arch = "x86", target_arch = "x86_64"))].

For instance when doing cross test --target aarch64-unknown-linux-gnu, target_arch in build.rs is x86_64, so it does not check for the correct target features. It looks like an universally complex topic. In the meantime I am also exploring the runtime feature resolution, but that would require a fallback, as we don't want the crate to build but to panic at runtime.

[ogxd]

Closed in favor of #61