sh: use opcode_t and enable unaligned code for sh2a

This patch converts the unaligned access handling code to use opcode_t
instead of u16. While at it, enable unaligned access handling for sh2a.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>