auto merge of #16258 : aturon/rust/stabilize-atomics, r=alexcrichton

This commit stabilizes the `std::sync::atomics` module, renaming it to
`std::sync::atomic` to match library precedent elsewhere, and tightening
up behavior around incorrect memory ordering annotations.

The vast majority of the module is now `stable`. However, the
`AtomicOption` type has been deprecated, since it is essentially unused
and is not truly a primitive atomic type. It will eventually be replaced
by a higher-level abstraction like MVars.

Due to deprecations, this is a:

[breaking-change]

diff --cc src/librustrt/unwind.rs
Simple merge

diff --cc src/libstd/rt/backtrace.rs
Simple merge