remove using_exec_ops global

remove using_exec_ops global

This removes the using_exec_ops global from exec.c, in favor of
querying the target stack directly using target_is_pushed.  This is
more in keeping with other code in gdb, and is also more future-proof
as it is more multi-target-ready.

Built and regtested on x86-64 Fedora 20.

2014-07-25  Tom Tromey  <tromey@redhat.com>

	* exec.c (using_exec_ops): Remove.
	(exec_close_1): Update.  Remove extraneous block, reindent.
	(add_target_sections): Use target_is_pushed.