Set thread pool thread name just based on string pointer check. (mono/mono#16637)

Set thread pool thread name just based on string pointer check. (mono/mono#16637)

* Set thread pool thread name just based on string pointer check.

When the generation-based solution went in, there were not
yet constant thread names, so a pointer based approach could fail
due to reuse. We can do slightly simpler now.

* Remove the generation field which is no longer needed.
Remove the duplicated knowledge of how to set a constant and put
it behind a funny sounding but perhaps reasonable flag.
Revise corlib version.

Commit migrated from https://github.com/mono/mono/commit/2129ac6c282560e3039a38de9ebbebcdfab6f149