printk: add support of hh length modifier for printk

Impact: new feature, extend vsprintf format strings

hh is used as length modifier for signed char or unsigned char.
It is supported by glibc, we add kernel support now.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Acked-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: torvalds@linux-foundation.org
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <49CC9739.30107@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>