Support architectures without atomics with polyfill

Cargo.toml

@@ -10,6 +10,7 @@ authors = ["Matti Virkkunen <[email protected]>"]
 repository = "https://github.com/mvirkkunen/usb-device"
 
 [dependencies]
+atomic-polyfill = "1.0.2"
 defmt = { version = "0.3", optional = true }
 
 [dev-dependencies]

src/bus.rs

@@ -3,10 +3,10 @@ use crate::endpoint::{
     IsochronousUsageType,
 };
 use crate::{Result, UsbDirection, UsbError};
+use atomic_polyfill::{AtomicPtr, Ordering};
 use core::cell::RefCell;
 use core::mem;
 use core::ptr;
-use core::sync::atomic::{AtomicPtr, Ordering};
 
 /// A trait for device-specific USB peripherals. Implement this to add support for a new hardware
 /// platform.

src/endpoint.rs

@@ -1,7 +1,7 @@
 use crate::bus::UsbBus;
 use crate::{Result, UsbDirection};
+use atomic_polyfill::{AtomicPtr, Ordering};
 use core::marker::PhantomData;
-use core::sync::atomic::{AtomicPtr, Ordering};
 
 /// Trait for endpoint type markers.
 pub trait EndpointDirection {