samples/synchronization: fix thread b pinning

This commit adds a brief thread b suspend while the sample sets its
affinity mask.

If the call to `k_thread_cpu_pin` is being made while the thread is
actively running, then we get `-EINVAL` and the affinity mask is left
unchanged.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>

samples/synchronization/src/main.c

@@ -110,7 +110,16 @@ int main(void)
 #if PIN_THREADS
 	if (arch_num_cpus() > 1) {
 		k_thread_cpu_pin(&thread_a_data, 0);
+
+		/*
+		 * Thread b is a static thread that is spawned immediately. This means that the
+		 * following `k_thread_cpu_pin` call can fail with `-EINVAL` if the thread is
+		 * actively running. Let's suspend the thread and resume it after the affinity mask
+		 * is set.
+		 */
+		k_thread_suspend(thread_b);
 		k_thread_cpu_pin(thread_b, 1);
+		k_thread_resume(thread_b);
 	}
 #endif