re-stabilize the AVX-512 features that were stabilized in Rust 1.27.0 (#842)
re-stabilize the AVX-512 features that were stabilized in Rust 1.27.0 (#842)

* re-stabilize the AVX-512 features that were stabilized in Rust 1.27.0

https://github.com/rust-lang/stdarch/pull/739 added per-feature
stabilization of runtime CPU feature detection. In so doing, it
de-stabilized some detection features that had been stable since Rust
1.27.0, breaking some published crates (on nightly). This commit
re-stabilizes the subset of AVX-512 detection features that were
included in 1.27.0 (that is, the pre-Ice-Lake subset). Other instruction
sets (MMX in particular) remain de-stabilized, pending a decision about
whether should ever stabilize them.

See https://github.com/rust-lang/rust/issues/68905.

* add a comment explaining feature detection stability

* adjust stabilizations to match most recent proposal

https://github.com/rust-lang/rust/issues/68905#issuecomment-595376319
rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch