printk: make it 64-bit compatible

On 64-bit systems the most notable difference is due to longs and
pointers being 64-bit wide. Therefore there must be a distinction
between ints and longs. Similar to the prf.c case, this patch properly
implements the h, hh, l, ll and z length modifiers as well as some small
cleanups.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>