zephyr:embassy: Use a semaphore for the executor

Fixes a potential race condition in the suspend/resume sequence.

Author: aolowin

zephyr/src/embassy/executor.rs

@@ -5,6 +5,8 @@ use core::marker::PhantomData;
 use crate::sys::sync::Semaphore;
 use crate::time::Forever;
 use embassy_executor::{raw, Spawner};
+use crate::sys::sync::Semaphore;
+use crate::time::Forever;
 
 /// Zephyr-thread based executor.
 pub struct Executor {