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>