Flush data cache for executable loadable segments explicitly.

Do not use textsize and do not flush everything between map base and
base + textsize, because unmapped areas cannot be flushed.

This makes Obj_Entry textsize only use go away, and I will remove it
later.

Reported by: tuexen
Tested by: Mark Millard <marklmi26-fbsd@yahoo.com>
Sponsored by: The FreeBSD Foundation