Make bfd.link_next field a union

This field of struct bfd is currently only used to chain together
linker input files.  This patch prepares to use the field to stash
the linker hash table, which is always created on the linker output
file.

bfd/
	* bfd.c (struct bfd): Replace link_next with a union.
	* aoutx.h, * bfd.c, * coff-ppc.c, * coff-rs6000.c, * cofflink.c,
	* ecoff.c, * elf-m10300.c, * elf32-arm.c, * elf32-avr.c,
	* elf32-hppa.c, * elf32-i386.c, * elf32-lm32.c, * elf32-m32c.c,
	* elf32-m32r.c, * elf32-m68hc1x.c, * elf32-metag.c,
	* elf32-microblaze.c, * elf32-nds32.c, * elf32-nios2.c,
	* elf32-or1k.c, * elf32-ppc.c, * elf32-rl78.c, * elf32-s390.c,
	* elf32-score.c, * elf32-score7.c, * elf32-sh.c, * elf32-spu.c,
	* elf32-tic6x.c, * elf32-tilepro.c, * elf32-xstormy16.c,
	* elf32-xtensa.c, * elf64-alpha.c, * elf64-hppa.c, * elf64-ia64-vms.c,
	* elf64-mmix.c, * elf64-ppc.c, * elf64-s390.c, * elf64-x86-64.c,
	* elflink.c, * elfnn-aarch64.c, * elfxx-mips.c, * elfxx-sparc.c,
	* elfxx-tilegx.c, * linker.c, * pdp11.c, * peXXigen.c, * simple.c,
	* sunos.c, * vms-alpha.c, * xcofflink.c: Update for above.
	* bfd-in2.h: Regenerate.
include/
	* bfdlink.h: Update for bfd.link_next change.
ld/
	* emultempl/cr16elf.em, * emultempl/elf32.em, * emultempl/genelf.em,
	* emultempl/m68kcoff.em, * emultempl/m68kelf.em,
	* emultempl/nds32elf.em, * emultempl/pe.em, * emultempl/pep.em,
	* ldlang.c, * ldmain.c, * pe-dll.c: Update for bfd.link_next change.

include/bfdlink.h

@@ -469,7 +469,7 @@ struct bfd_link_info
   bfd *output_bfd;
 
   /* The list of input BFD's involved in the link.  These are chained
-     together via the link_next field.  */
+     together via the link.next field.  */
   bfd *input_bfds;
   bfd **input_bfds_tail;