Fix remaining Ravenscar regressions

Testing showed a few more Ravenscar regressions arising from upstream.
In particular, gdb now uses the current thread in some places where
inferior_ptid was previously used.  This patch fixes the problem by
arranging to save and restore the thread now.

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

	* ravenscar-thread.c
	(ravenscar_thread_target::set_base_thread_from_ravenscar_task):
	New method.
	(ravenscar_thread_target::wait): Check
	runtime_initialized.
	(ravenscar_thread_target::prepare_to_store)
	(ravenscar_thread_target::stopped_by_sw_breakpoint)
	(ravenscar_thread_target::stopped_by_hw_breakpoint)
	(ravenscar_thread_target::stopped_by_watchpoint)
	(ravenscar_thread_target::stopped_data_address)
	(ravenscar_thread_target::core_of_thread): Use
	scoped_restore_current_thread and
	set_base_thread_from_ravenscar_task.