verilator_tb_utils: change all 'times' to uint64_t

Franck Jullien reported that unsigned long is not a valid argument
for tfp->dump() on 32-bit machines.
This changes all 'time' (cycles, really) references to uint64_t.
Merge pull request #48 from skristiansson/verilator_tb_utils

verilator_tb_utils: change all 'times' to uint64_t