Stabilize vget_low_s8 on AArch64

[Amanieu]

This was missed when most of the AArch64 intrinsics were stabilized.

cc rust-lang/rust#94083

[rust-highfive]

@Amanieu: no appropriate reviewer found, use r? to override

[sugar700]

This probably shouldn't be 1.59.0 as it's probably not going to be backported.

[Amanieu]

I tried that but rustc doesn't like it when I put different version numbers on the same feature name.

[sugar700]

I think you can use a different feature name such as vget_low_s8, then rustc won't complain. Feature names for stable features are mostly irrelevant anyway - those features are stable, they don't need feature annotation. Stabilizing things while changing their feature names was done before, see rust-lang/rust#86344 for instance (in this case, this stabilized MaybeUninit::write which back then used maybe_uninit_extra feature without stabilizing everything else that used maybe_uninit_extra feature).