libbb: use a wrapper around sysconf(_SC_CLK_TCK) to save a few bytes

libbb: use a wrapper around sysconf(_SC_CLK_TCK) to save a few bytes

function                                             old     new   delta
bb_sc_clk_tck                                          -      10     +10
timescmd                                             118     113      -5
print_route                                         1763    1758      -5
mpstat_main                                         1288    1283      -5
iostat_main                                         1947    1942      -5
INET_setroute                                        879     871      -8
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/5 up/down: 10/-28)            Total: -18 bytes
Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>