Make the ravenscar thread target multi-target-ready

Make the ravenscar thread target multi-target-ready

This changes ravenscar-thread.c to make it ready for multi-target.
This is done by moving globals into the target, and then arranging to
allocate the target with "new" and delete the target in its "close"
method.

gdb/ChangeLog
2019-02-15  Tom Tromey  <tromey@adacore.com>

	* ravenscar-thread.c (base_ptid): Remove.
	(struct ravenscar_thread_target) <close>: New method.
	<m_base_ptid>: New member.
	<update_inferior_ptid, active_task, task_is_currently_active,
	runtime_initialized>: Declare methods.
	<ravenscar_thread_target>: Add constructor.
	(ravenscar_thread_target::task_is_currently_active)
	(ravenscar_thread_target::update_inferior_ptid)
	(ravenscar_runtime_initialized): Rename.  Now methods.
	(ravenscar_thread_target::resume, ravenscar_thread_target::wait)
	(ravenscar_thread_target::update_thread_list): Update.
	(ravenscar_thread_target::active_task): Now method.
	(ravenscar_thread_target::store_registers)
	(ravenscar_thread_target::prepare_to_store)
	(ravenscar_thread_target::prepare_to_store)
	(ravenscar_thread_target::mourn_inferior): Update.
	(ravenscar_inferior_created): Use "new" to create target.
	(ravenscar_thread_target::get_ada_task_ptid): Update.
	(_initialize_ravenscar): Don't initialize base_ptid.
	(ravenscar_ops): Remove global.