Fix ravenscar-thread.c to use arch_ops

Fix ravenscar-thread.c to use arch_ops

The change to turn target ops into methods accidentally introduced a
bug in ravenscar-thread.c, changing some calls that were using
"arch_ops" to use the target beneath.

This patch changes ravenscar-thread.c to use these variables where
appropriate.

gdb/ChangeLog
2018-07-22  Tom Tromey  <tom@tromey.com>

	* ravenscar-thread.c (ravenscar_thread_target::store_registers):
	Use arch_ops.
	(ravenscar_thread_target::prepare_to_store): Likewise.