clock: Use monotonic clock in the thread scheduler

Switch the thread scheduler from using the s64 type for durations and
the wall time, to the osv::clock::uptime::duration type (which is
std::chrono::nanoseconds) and monotonic clock.

Also, now that the per-thread CPU-time clock (thread::thread_clock())
returns an std::chrono::duration instead of s64, we no longer need the
fill_ts(s64) variant in libc/time.cc (if we leave it unused, we'll get a
compilation warning).

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>