Rollup merge of #154950 - davidtwco:scalable-no-cfg, r=RalfJung

library: no `cfg(target_arch)` on scalable intrinsics

These intrinsics don't technically need to be limited to a specific architecture, they'll probably only make sense to use on AArch64, but this just makes it harder to use them in stdarch where it is appropriate (such as on `arm64ec`): requiring a rustc patch to land and be on nightly before stdarch work can proceed. So let's just not `cfg` them at all, they're perma-unstable anyway.

Fixes CI failure in rust-lang/stdarch#2071

Author: JonathanBrouwer