Atomic polyfill

[Urhengulas]

This PR replaces core::sync::atomic::* with atomic_polyfill::*.

This change makes defmt-rtt available on targets without native atomics.
Fixes #597.

There should be no negative impact on targets with atomic support, since in these cases the native atomics will be used.

[Urhengulas]

We decided to pause this change until we do a coordinated effort to get risc-v support for defmt and probe-run. All the relevant issues will be tracked with the tag topic: risc-v.

[TheButlah]

Thats unfortunate given that the change is a minor one. I'll continue to use this code in a fork and patch my dependencies, I guess

[Urhengulas]

@TheButlah said:

Thats unfortunate given that the change is a minor one. I'll continue to use this code in a fork and patch my dependencies, I guess

Is defmt + defmt-rtt really usable on esp32c3 with this change? Isn't esp32c3 support anyways blocked on #693? Not even talking about probe-run which supports risc-v even less.

[TheButlah]

Yes, this PR fully fixes demt on the esp32c3 when compiled for -imc. I have been successfully using defmt for a while, albeit on the -imac target (I had to change because the atomic trap handler was broken). I have switched to -imc due to performance reasons and because of a bug in the atomic trap handler (the esp32c3 doesn't natively support atomics)

The linker script works fine for direct-boot mode, which is what most people using esp-hal use anyway. Pretty sure it would work for booloader mode too, but no one bothered to try it.

probe-run doesn't work, but cargo embed and the other probe-rs tooling does.

Also this PR isn't really about riscv, its more about supporting targets that don't have atomics