feat: add atomic-polyfill to support more targets

[elpiel]

Fixes #9

CI builds for those targets can also be added and mentioned in README.
critical-section implementation is also required.

[avsaase]

What is needed to get this PR merged?

[elpiel]

@avsaase you could as in the Matrix channel

[tommasoclini]

Is this going to be merged?

[elpiel]

I don't think there are any maintainers anymore.
If needed at-me because I'm unsubscribing from the PR

[robin-nitrokey]

I’m reluctant to pull in an additional dependency unconditionally but I think it should be fine to have a feature flag that replaces core::sync::atomic with atomic_polyfill (or rather portable_atomic). @sosthene-nitrokey What do you think?

[sosthene-nitrokey]

Adding portable_atomic would be better than atomic_polyfill. I think the dependency should be kept optional. It's not that much more code to do so.

[robin-nitrokey]

Closing in favor of #25.