Make bfd.link_next field a union

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.