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

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Amanieu merged 3 commits intorust-lang:masterfrom

oconnor663:restabilize

Mar 19, 2020

Commits on Feb 27, 2020

re-stabilize the AVX-512 features that were stabilized in Rust 1.27.0
oconnor663
committed
add a comment explaining feature detection stability
oconnor663
committed

Commits on Mar 11, 2020

adjust stabilizations to match most recent proposal
oconnor663
committed