Fix AO_pause sleep delay for particular argument values (Win32)

Fix AO_pause sleep delay for particular argument values (Win32)

* src/atomic_ops.c (AO_pause): Fix millis value (passed to Win32
Sleep) for the case of 'n' argument in range between 12 and 21,
inclusive if AO_USE_WIN32_PTHREADS (sleep for 1 ms in this case).
* src/atomic_ops_stack.c (AO_pause): Evaluate 'msecs' value using the
same algorithm as in atomic_ops.c (Win32 only).