Update Ticks64() to use monotonic clock on Unix systems

```
On non-Darwin Unix systems, Ticks64() uses gettimeofday(), which returns a wall 
clock date/time.  On POSIX platforms that support a monotonic clock via 
clock_gettime(), that should be used instead.
```

Original issue reported on code.google.com by `gambit47` on 21 Jul 2015 at 10:19
