sh2(A) exception handler update

This patch is

By sh2
- Remove duplicate code
- Reduce stack usage
- Cleanup and little optimize

By sh2a
- Add missing handler(256 to 511)
- Use sh2a instructions handler

Signed-off-by: Yoshinori Sato <ysato@users.sourceforge.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>