Add mono_gc_thread_detach callback. (mono/mono#18408)

Add mono_gc_thread_detach callback. (mono/mono#18408)

Add mono_gc_thread_detach callback for Boehm

Use under Boehm to detach without threads lock as Boehm will take it's own lock internally.

Note in `on_gc_notification` we take threads lock after Boehm already has it's own lock. For consistency and to prevent deadlocks always take lock ordering of Boehm then threads lock.

Commit migrated from https://github.com/mono/mono/commit/480bef72172856bd0f1a57243ddb53e98684f7d1