heapless does not build for riscv32imc-unknown-none-elf target

[rjoleary]

rv32imc does not have atomics leading to this issue:

error[E0599]: no method named `compare_exchange_weak` found for type `core::sync::atomic::AtomicPtr<pool::Node<T>>` in the current scope
   --> /home/ryan/.cargo/registry/src/github.com-1ecc6299db9ec823/heapless-0.4.4/src/pool/mod.rs:278:33
    |
278 |                 match self.head.compare_exchange_weak(
    |                                 ^^^^^^^^^^^^^^^^^^^^^ method not found in `core::sync::atomic::AtomicPtr<pool::Node<T>>`

[jamesmunns]

I believe this would be closed by #120.

[japaric]

I believe this would be closed by #120.

That PR is not sufficient. That target does not have atomic loads either so AtomicUsize et al. are missing from core meaning that the spsc module doesn't compile for this target even after disabling the "cas" Cargo feature.