shootout-spectralnorm tweaks

- using libgreen to optimize CPU usage
- less tasks to limit wasted resources

Here, on a one core 2 threads CPU, new version is ~1.2 faster.  May
be better with more core.
auto merge of #13921 : TeXitoi/rust/shootout-spectralnorm-tweaks, r=alexcrichton

- using libgreen to optimize CPU usage
- less tasks to limit wasted resources

Here, on a one core 2 threads CPU, new version is ~1.2 faster.  May
be better with more core.