Support targets that lack atomics

[ealasu]

This crate fails to compile on targets that lack atomic primitives, like thumbv6m-none-eabi. Would it make sense to support such targets? One option might be to add a feature called "use_atomic" that's enabled by default, and add #[cfg(feature = use_atomic)] to all the modules that need atomics.

[alexcrichton]

With the closing of #627 I'm going to close this as well

[notgull]

I recently ran into this issue while trying to test out the event-listener crate on Arduino. We may be able to use either portable-atomic or atomic-polyfill to replace the atomics, at least on the lower-level crates (like futures-core, which is the crate that's causing my issue).

[taiki-e]

I would accept a patch to support the portable-atomic.

[notgull]

Opened #2670