[POWERPC] fix showing xmon help

In some configuration, xmon help string is larger than xmon_printf
buffer.  We need not to use printf.  This patch adds xmon_puts and
change to use it to show help string.

Signed-off-by: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Signed-off-by: Paul Mackerras <paulus@samba.org>