clock: Use new clock APIs in msleep implementation

Fix msleep implementation to use the new <osv/clock.hh> APIs and the
monotonic clock.

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>

bsd/porting/synch.cc

@@ -86,8 +86,7 @@ int synch_port::_msleep(void *chan, struct mtx *mtx,
 
     if (timo_hz) {
         u64 nanoseconds = ticks2ns(timo_hz);
-        u64 cur_time = clock::get()->time();
-        t.set(cur_time + nanoseconds);
+        t.set(std::chrono::nanoseconds(nanoseconds));
     }
 
     trace_synch_msleep_wait(chan, timo_hz);