fix calls to find_target_beneath

A long time ago Pedro pointed out that there are some calls to
find_target_beneath that pass in an explicit target_ops; but which
should instead use the ops provided to the method in question.  See:

    https://sourceware.org/ml/gdb-patches/2014-01/msg00429.html

This patch is just a minor cleanup to fix all such calls.  There were
only three.

2014-07-18  Tom Tromey  <tromey@redhat.com>

	* spu-multiarch.c (spu_region_ok_for_hw_watchpoint): Pass "self"
	to find_target_beneath.
	* ravenscar-thread.c (ravenscar_prepare_to_store): Pass "ops" to
	find_target_beneath.
	(ravenscar_mourn_inferior): Pass "self" to find_target_beneath.