sh: add byte support to the sign extension code

This patch adds byte support to the sign extension code. Unaligned access
traps should never be generated on 8-bit io operations, but we will use this
code for trapped io and we do need byte support there.

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