[netcore] Managed ThreadPool implementation (mono/mono#17387)

[netcore] Managed ThreadPool implementation (mono/mono#17387)

* Wire up the managed side of the portable thread pool

* Remove old native thread pool code

* Fix non-netcore build

* Fix Thread.IsThreadPoolThread

* Fix MSVC builds

* Code style fix

* Make the locking primitives uninterruptible

* Revert icall changes for non-netcore

* Implement Thread.ResetThreadPoolThread

* Compile out more thread pool code for netcore

* Remove ThreadPoolBoundHandle, it is shared now

* Address PR feedback on code style

Commit migrated from https://github.com/mono/mono/commit/7a15b718735eccfac6ecd7467a09279d2f23e16e