fix AVR support by using atomic-polyfill

[mutantbob]

Attempting to compile a project for AVR that depends on ahash fails:

error[E0432]: unresolved import `atomic::AtomicUsize`
  --> /home/thoth/.cargo/registry/src/github.com-1ecc6299db9ec823/once_cell-1.13.1/src/race.rs:27:14
   |
27 | use atomic::{AtomicUsize, Ordering};
   |              ^^^^^^^^^^^ no `AtomicUsize` in `sync::atomic`

error[E0432]: unresolved import `super::atomic::AtomicPtr`

The once_cell crate has an atomic-polyfill feature that can be enabled to solve this problem, and we adapt its technique to solve ahash's dependencies on core::sync::atomic .