[runtime] Add a mono_thread_detach_if_exiting () public api function which can be called by embedding code to detach the runtime if the code is running from a pthread dtor. Fixes part of #21164.
[rpm spec] Mono.Web and OpenSystem.C do not exist anymore
Merge pull request #1355 from echampet/back-3.6

backport spec file fix to 3.6