+stable build on M1 Mac

[eadf]

Basically the same problem as #12.

$ cargo +stable build

   Compiling bitintr v0.3.0 (/Users/redacted/Develop/rust-workspace/bitintr)
error[E0658]: use of unstable library feature 'stdsimd'
  --> src/lib.rs:28:13
   |
28 |     pub use core::arch::aarch64::*;
   |             ^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #27731 <https://github.com/rust-lang/rust/issues/27731> for more information

Does this crate (intentionally) require +nightly on the aarch64 platform?

$ rustc +stable --version
rustc 1.55.0 (c8dfcfe04 2021-09-06)