change delegation for to_read_description

change delegation for to_read_description

This switches to_read_description to the "new normal" delegation
scheme.  This one was a bit trickier than the other changes due to the
way that target_read_description handled delegation.  I examined all
the target implementations of to_read_description and changed the ones
returning NULL to instead delegate.

2014-02-19  Tom Tromey  <tromey@redhat.com>

	* arm-linux-nat.c (arm_linux_read_description): Delegate when
	needed.
	* corelow.c (core_read_description): Delegate when needed.
	* remote.c (remote_read_description): Delegate when needed.
	* target-delegates.c: Rebuild.
	* target.c (target_read_description): Rewrite.
	* target.h (struct target_ops) <to_read_description>: Update
	comment.  Use TARGET_DEFAULT_RETURN.