Fix AArch64 build when the nightly feature is enabled

[AlexTMjugador]

rust-lang/stdarch#1573 stabilized the stdarch_arm_crc32 feature, meaning that it's no longer necessary to enable it to get ARM CRC intrinsics, and doing so in the latest nightlies causes compile errors to happen:

Let's get rid of that removed unstable feature and update the documentation accordingly.

[srijs]

Hi, thanks for the PR! It's great to see that the feature is finally stabilized.

However, the main issue that I see is that removing the nightly guard completely will mean that older rust version (our MSRV is currently 1.46) will then fail to build on aarch64.

I've just removed our version detection in fb5bf16, which I'm open to bring back to guard this feature for rust >= 1.80, but I'm also open to other suggestions!

[srijs]

I've published a short term fix in 1.4.2 to make the nightly feature compile again, but what's still missing is moving the aarch64 CRC32 feature from behind the nightly flag to general availability for Rust version >= 1.80.

[AlexTMjugador]

Hi, thanks for the review!

I made the PR in a bit of a hurry and didn't think about older stable Rust version support, sorry.

That being said, if we don't want to bump MSRV at the moment, I think the best we can do is bringing back some form of rustc version detection, to only enable AArch64 SIMD intrinsics on versions that have this feature stabilized. Would you mind if I repurpose this PR to do exactly that?

[srijs]

No worries - yep that sounds fine, let's use this PR then for the proper fix 👍

Feel free to copy most of the old code that got removed in fb5bf16 and adapt it, in general I'd prefer not adding another dependency just for this.