Use atomic polyfill for targets missing atomic support

[MabezDev]

Drops in emulated atomics for targets missing core::sync::atomic, on targets that do not have them natively.

[bjoernQ]

I'd be quite interested in getting this into rtt-target (to make RTT work on BL602)

[MabezDev]

@bjoernQ, this may be of interest: https://github.com/esp-rs/riscv-atomic-emulation-trap.

[noppej]

@mvirkkunen What are your thoughts on enabling RISCV targets such as the ESP32C3 board?

[Dirbaio]

We shouldn't do this. This crate only needs load/store, not CAS. atomic_polyfill will take a critical section for each load/store to prevent them from racing against CAS operations, which is useless overhead in this case because there's no CAS.

Ideally Rust should expose load/store atomics on riscv32-imc (what the esp32c3 is) like it does in thumbv6m. See relevant Rust issue: rust-lang/rust#99668

[MabezDev]

Oh wow, I forgot this PR even existed :D. I agree and I think it's coming soon with the +forced-atomics feature. FYI it's also possible to "force" them on the rust side via cfg flags, see https://github.com/esp-rs/riscv-atomic-emulation-trap#usage.