Add benchmarks for thread pool throughput

[filipnavara]

There are several benchmarks floating around that would be nice to have in regular performance runs:

• Add normalized equivalent of YieldProcessor, retune some spin loops coreclr#13670 (comment)
• https://gist.github.com/filipnavara/8e6aa10131e68d313934b006aea82abd (BDN rewrite of some of the benchmarks from link above)
• https://github.com/benaadams/ThreadPoolTaskTesting

[adamsitnik]

Good idea!