Don't write to inferior_ptid in ravenscar-thread.c

gdb/ChangeLog:
2020-06-18  Pedro Alves  <palves@redhat.com>

	* ravenscar-thread.c (ravenscar_thread_target): Update.
	(ravenscar_thread_target::update_inferior_ptid): Rename to ...
	(ravenscar_thread_target::add_active_thread): ... this.  Don't
	set m_base_ptid here.  Update to avoid referencing inferior_ptid.
	(ravenscar_thread_target::wait): Don't write to inferior_ptid.