Call add_active_thread after pushing the ravenscar target

Currently ravenscar-thread.c calls add_active_thread before pushing
the ravenscar target.  This yields an initial thread announcement of
"[Thread 0]".  Calling add_active_thread after pushing the target
fixes this.

gdb/ChangeLog
2020-08-07  Tom Tromey  <tromey@adacore.com>

	* ravenscar-thread.c (ravenscar_thread_target): Don't call
	add_active_thread.
	(ravenscar_thread_target::add_active_thread): Now public.
	(ravenscar_inferior_created): Call add_active_thread after pushing
	the target.