Add relaxed-atomics feature flag (#8983)

We previously renamed the old "relaxed-atomics" feature to
"acquire-release-atomics." Add an experimental "relaxed-atomics" feature
that will allow "relaxed" atomics in addition to seqcst and
acquire-release atomics. The "relaxed" atomics will actually be
load-store ordered atomics, which prevent out-of-thin-air values for a
(hopefully) modest performance cost. The name of the feature is likely
to change in the future to more exactly represent the desired semantics.