Executor: Replace unnecessary atomics in runqueue

[GrantM11235]

Only the head pointer needs to be atomic. The RunQueueItem pointers are only loaded and stored, and never concurrently

[Dirbaio]

This could use the core (not atomic-polyfill) AtomicPtr instead. It has no overhead for loads/stores, and there's less unsafe.

The only downside is it's not available for riscv, but that should get solved soon.

[GrantM11235]

I don't think that using AtomicPtr is actually any safer in this case. Any bug that could cause a data race here would almost certainly also lead to UB somewhere else. I think that using an atomic here actually makes it harder to reason about, because it falsely implies that there can/will be concurrent accesses.

[Dirbaio]

Fair! SGTM1 Also using SyncUnsafeCell allows using Option which is nice.

bors r+

[bors]

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