Use symbolic constants instead of magic numbers with add_data_entry()

Use symbolic constants instead of magic numbers with add_data_entry()

Use symbolic constants from include/coff/internal.h instead of magic numbers
with add_data_entry()

bfd/ChangeLog:

2017-04-02  Jon Turney  <jon.turney@dronecode.org.uk>

	(_bfd_XXi_swap_aouthdr_out): For clarity, use defines rather than
	numbers for DataDirectory entry indicies passed to
	add_data_entry().