linux/threads: simplify timer code

Possibly also changes the semantics. According to the docs, it should now be
equal to the Windows code (up to accuracy issues, of course).

ping @gregory38 

This is actually more of an alternative implementation so that I can learn and find out why the current implementation is the way it is.
I don't understand why you change the formula. Save a multiplication? Code is much more easier to read if you scale it to GetThreadTicksPerSecond.

Note: actually it can be written as
```
(ts.tv_sec + ts.tv_nsec / 1e9) * GetThreadTicksPerSecond();
```
So function return a number of tick unit (Not unit of 100 ns)