preempt: annotate for branch prediction
preempt: annotate for branch prediction

We can predict that need_preempt() will usually return false, helping
the compiler push the prempted path out of the hot path.
Message-Id: <4c7414d1e91c5ee0c8158e181cc06e151b6fcfee@scylladb.com>

syuu1228/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,scylladb/seastar,scylladb/seastar,dreamsxin/seastar,dreamsxin/seastar,avikivity/seastar,cloudius-systems/seastar,cloudius-systems/seastar,syuu1228/seastar,avikivity/seastar,dreamsxin/seastar,cloudius-systems/seastar