maitake-sync: add fair spinlock

[hawkw]

maitake-sync's async Mutex and RwLock are guaranteed to be fair, because they use FIFO wait queues. however, the spin module's blocking locks are not fair. it would be nice to have a fair FIFO spinlock in maitake-sync, in addition to our current unfair mutex. we could implement a fair spinlock using a ticket lock.