Fix printing of 64-bit counters on 32-bit ppc platforms.

Several statistic counters are uint64_t values and are printed by systat
using %lu. This results in displaying wrong numbers. Use PRIu64 instead.
While there, print variables of size_t using %zd.

MFC after:i 3 days
Differential Revision: https://reviews.freebsd.org/D17838