Nikkei hits 21-month high, led by exporters on weaker yen

* Yen weakness boosts exporters * Nikkei up 23 percent this year * Yen falls 12.2 percent vs dollar * Nikkei, Topix rise 0.7 pct * Long-only funds' buying continues - analyst By Ayai Tomisawa TOKYO, Dec 28 (Reuters) - Japan's Nikkei average climbed to a 21-month high on Friday, posting its best yearly gain since 2005, as the yen weakened further on rising expectations of aggressive monetary stimulus under new Prime Minister Shinzo Abe. Exporters were in demand, with