Use atomic polyfill for core::sync::atomic.

[royb3]

Hi, I ran into an issue with building embassy for the riscv32i-unknown-none-elf target.

This target's core lib does not contain sync::atomic, or it's empty. The imports to atomic-polyfill fixes the issue, and has been done for other parts of Embassy too.

[Dirbaio]

I'm sorry, but this is not the right fix for the problem. See previous discussion in #1240, #745 (comment), rust-lang/rust#99668

[royb3]

Thanks for pointing me in that direction. For now we seems to have a workaround. Our Risc V implementation actually has the I A and M extensions, but the C extension is not implemented.

Copying these rustflags from https://github.com/esp-rs/riscv-atomic-emulation-trap, and building the core lib with -Zbuild-std=core seems to do the trick

[target.riscv32i-unknown-none-elf]
rustflags = [
  "-C", "link-arg=-Tmemory.x",
  "-C", "link-arg=-Tlink.x",
  "-C", "link-arg=-Tdefmt.x",
  "-C", "target-feature=+m",
  "-C", "target-feature=+a",
  # tell the core library have atomics even though it's not specified in the target definition
  "--cfg", "target_has_atomic_load_store",
  "--cfg", 'target_has_atomic_load_store="8"',
  "--cfg", 'target_has_atomic_load_store="16"',
  "--cfg", 'target_has_atomic_load_store="32"',
  "--cfg", 'target_has_atomic_load_store="ptr"',
  # enable cas
  "--cfg", "target_has_atomic",
  "--cfg", 'target_has_atomic="8"',
  "--cfg", 'target_has_atomic="16"',
  "--cfg", 'target_has_atomic="32"',
  "--cfg", 'target_has_atomic="ptr"',
]