prf.c: implement the ll length modifiers

This allows for printing long long values. Because the code size
increase may be significant, this is made optional on 32-bit targets.
On 64-bit targets this doesn't change the code much as longs and
long longs are the same size so it is always enabled in that case.

The test on MAXFLD has to be adjusted accordingly. Yet, its minimum
value wasn't large enough to store a full-scale octal value, so this
is fixed as well.

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