use std::chrono::system_clock::now() for random seed

[dwrensha]

Currently, the default random seed is set by time(NULL), which returns an integer number of seconds. If we call llama_sampling_init() many times within a single second, then all of the sampling contexts get the same random seed. Such behavior can come as an unpleasant surprise. I encountered it today while attempting to generate many samples via ollama, which makes use of an adapted version of llama.cpp/examples/server/server.cpp.

We can avoid the problem by using a higher-precision clock to set the random seed. On my (Linux) system, std::chrono::system_clock measures its ticks in nanoseconds. On Windows/Visual Studio, a tick is 100 nanoseconds. static_cast<uint32_t> then truncates the higher-order bits.

An alternative to this clock-based approach would be to use std::random_device, as is already done in utils.hpp. I opted against that approach in this PR, however, because, as noted here, on some platforms std::random_device is deterministic.

[dwrensha]

Closing in favor of #6962, which seems nicer to me. Please feel free to reopen and accept this one instead, if you prefer. I consider this one to be a more conservative fix, but slightly less nice.