[OpenMP][OMPT] Add missing callbacks for asynchronous target tasks (#93472)

- The first hidden-helper-thread did not trigger thread-begin
- The "detaching" from a target-task when waiting for completion missed
to call task-switch
- Target tasks identified themself as explicit task

Co-authored-by: Kaloyan Ignatov <kaloyan.ignatov@rwth-aachen.de>