samples/synchronization: Clean up SMP CPU pinning example

There was some code to demonstrate the cpu_mask API here, but it was
asymmetric (only thread B was pinned) and assumed exactly two CPUs.

Start both threads pinned to CPUs 0 and 1 from an external main, and
predicate the pinning on there actually being more than one SMP CPU.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>