target_ops::beneath -> target_ops::beneath()

target_ops::beneath -> target_ops::beneath()

This changes target_ops::beneath from a pointer to a method, and
adjusts all references throughout.  The idea here is to make it easier
to change the target stack representation from an intrusive singly
linked list to something else without leaking implementation details
throughout.

The commit does not change the representation yet, that will be done
in a following patch.  That is why a new target_ops::m_beneath field
appears here.  That new field isn't really documented properly or made
private, simply because it will be removed shortly.

Note that target_ops::beneath() is essentially the same as the current
find_target_beneath routine.  The following patch will eliminate the
latter.

gdb/ChangeLog:
2018-06-07  Pedro Alves  <palves@redhat.com>

	* target.h (target_ops) <beneath>: Now a method.  All references
	updated.
	(target_ops) <m_beneath>: New.
	* target.c (target_ops::beneath): New.
	* corelow.c: Adjust all references to target_ops::beneath.
	* linux-thread-db.c: Likewise.
	* make-target-delegates: Likewise.
	* record-btrace.c: Likewise.
	* record-full.c: Likewise.
	* remote.c: Likewise.
	* target.c: Likewise.
	* target-delegates.c: Regenerate.