smp: specify this_shard_id as noexcept
smp: specify this_shard_id as noexcept

`internal::this_shared_id_ptr` and `this_shared_id` that calls it
are trivially noexcept.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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