util/backtrace: Mark noexcept if func() doesn't throw
util/backtrace: Mark noexcept if func() doesn't throw

Message-Id: <d197761ed9d23a2f95e33ad7b0b4d3b1bfab0f28@scylladb.com>

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