convert exec target to to_xclose

This converts the exec target to use to_xclose, to make it suitable
for multi-target.

2014-07-29  Tom Tromey  <tromey@redhat.com>

	* exec.c (exec_xclose_1): Rename from exec_close_1.  Free "self".
	(add_target_sections): Push a new target instance.
	(section_table_read_available_memory): Don't refer to "exec_ops".
	(set_section_command): Likewise.
	(init_exec_ops): Set to_xclose, not to_close.