Remove unnecessary use of atomic-polyfill.

Only use it when CAS is actually needed.

embassy-executor/src/arch/riscv32.rs

@@ -1,7 +1,6 @@
 use core::marker::PhantomData;
 use core::ptr;
-
-use atomic_polyfill::{AtomicBool, Ordering};
+use core::sync::atomic::{AtomicBool, Ordering};
 
 use super::{raw, Spawner};
 

embassy-executor/src/arch/xtensa.rs

@@ -1,7 +1,6 @@
 use core::marker::PhantomData;
 use core::ptr;
-
-use atomic_polyfill::{AtomicBool, Ordering};
+use core::sync::atomic::{AtomicBool, Ordering};
 
 use super::{raw, Spawner};