Yield fix

[hamadmarri]

When Task t1 calls yield, a yield mark is set to its vruntime on the most sig. bit to make it large, and it gets reset when another task gets picked.

However, in case there is no other tasks except t1 in the rq, no need to mark it with the yield bit.

[hamadmarri]

2- The yield bit on vruntime is bad since in cacule IS two tasks with yield bet get the same IS due to the big value of vruntime. It's better to be replaced with another method.