[PATCH] ppc64: print negative numbers correctly in boot wrapper

if num has a value of -1, accessing the digits[] array will fail and the
format string will be printed in funny way, or not at all. This happens if
one prints negative numbers.
Just change the code to match lib/vsprintf.c
asm/div64.h cant be used because u64 maps to u32 for this build.

Signed-off-by: Olaf Hering <olh@suse.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>