Rename u.weakdef and make it a circular list

Rename u.weakdef and make it a circular list

This makes the elf_link_hash_entry weakdef field, currently used to
point from a weak symbol to a strong alias, a circular list so that
all aliases can be found from any of them.  A new flag, is_weakalias,
distinguishes the weak symbol from a strong alias, and is used in all
places where we currently test u.weakdef != NULL.

With the original u.weakdef handling it was possible to have two or
more weak symbols pointing via u.weakdef to a strong definition.
Obviously that situation can't map to a circular list; One or more of
the weak symbols must point at another weak alias rather than the
strong definition.  To handle that, I've added an accessor function to
return the strong definition.

	* elf-bfd.h (struct elf_link_hash_entry): Add is_weakalias.
	Rename u.weakdef to u.alias and update comment.
	(weakdef): New static inline function.
	* elflink.c (bfd_elf_record_link_assignment) Test is_weakalias
	rather than u.weakdef != NULL, and use weakdef function.
	(_bfd_elf_adjust_dynamic_symbol): Likewise.
	(_bfd_elf_fix_symbol_flags): Likewise.  Clear is_weakalias on
	all aliases if def has been overridden in a regular object, not
	u.weakdef.
	(elf_link_add_object_symbols): Delete new_weakdef flag.  Test
	is_weakalias and use weakdef.  Set is_weakalias and circular
	u.alias.  Update comments.
	(_bfd_elf_gc_mark_rsec): Test is_weakalias rather than
	u.weakdef != NULL and use weakdef function.
	* elf-m10300.c (_bfd_mn10300_elf_adjust_dynamic_symbol): Test
	is_weakalias rather than u.weakdef != NULL and use weakdef
	function.  Assert that def is strong defined.
	* elf32-arc.c (elf_arc_adjust_dynamic_symbol): Likewise.
	* elf32-arm.c (elf32_arm_adjust_dynamic_symbol): Likewise.
	* elf32-bfin.c (elf32_bfinfdpic_adjust_dynamic_symbol): Likewise.
	(bfin_adjust_dynamic_symbol): Likewise.
	* elf32-cr16.c (_bfd_cr16_elf_adjust_dynamic_symbol): Likewise.
	* elf32-cris.c (elf_cris_adjust_dynamic_symbol): Likewise.
	* elf32-frv.c (elf32_frvfdpic_adjust_dynamic_symbol): Likewise.
	* elf32-hppa.c (elf32_hppa_adjust_dynamic_symbol): Likewise.
	* elf32-i370.c (i370_elf_adjust_dynamic_symbol): Likewise.
	* elf32-lm32.c (lm32_elf_adjust_dynamic_symbol): Likewise.
	* elf32-m32r.c (m32r_elf_adjust_dynamic_symbol): Likewise.
	* elf32-m68k.c (elf_m68k_adjust_dynamic_symbol): Likewise.
	* elf32-metag.c (elf_metag_adjust_dynamic_symbol): Likewise.
	* elf32-microblaze.c (microblaze_elf_adjust_dynamic_symbol): Likewise.
	* elf32-nds32.c (nds32_elf_adjust_dynamic_symbol): Likewise.
	* elf32-nios2.c (nios2_elf32_adjust_dynamic_symbol): Likewise.
	* elf32-or1k.c (or1k_elf_adjust_dynamic_symbol): Likewise.
	* elf32-ppc.c (ppc_elf_adjust_dynamic_symbol): Likewise.
	* elf32-s390.c (elf_s390_adjust_dynamic_symbol): Likewise.
	* elf32-score.c (s3_bfd_score_elf_adjust_dynamic_symbol): Likewise.
	* elf32-score7.c (s7_bfd_score_elf_adjust_dynamic_symbol): Likewise.
	* elf32-sh.c (sh_elf_adjust_dynamic_symbol): Likewise.
	* elf32-tic6x.c (elf32_tic6x_adjust_dynamic_symbol): Likewise.
	* elf32-tilepro.c (tilepro_elf_gc_mark_hook): Likewise.
	(tilepro_elf_adjust_dynamic_symbol): Likewise.
	* elf32-vax.c (elf_vax_adjust_dynamic_symbol): Likewise.
	* elf32-xtensa.c (elf_xtensa_adjust_dynamic_symbol): Likewise.
	* elf64-alpha.c (elf64_alpha_adjust_dynamic_symbol): Likewise.
	* elf64-hppa.c (elf64_hppa_adjust_dynamic_symbol): Likewise.
	* elf64-ia64-vms.c (elf64_ia64_adjust_dynamic_symbol): Likewise.
	* elf64-ppc.c (ppc64_elf_gc_mark_hook): Likewise.
	(ppc64_elf_adjust_dynamic_symbol): Likewise.
	* elf64-s390.c (elf_s390_adjust_dynamic_symbol): Likewise.
	* elf64-sh64.c (sh64_elf64_adjust_dynamic_symbol): Likewise.
	* elfnn-aarch64.c (elfNN_aarch64_adjust_dynamic_symbol): Likewise.
	* elfnn-ia64.c (elfNN_ia64_adjust_dynamic_symbol): Likewise.
	* elfnn-riscv.c (riscv_elf_adjust_dynamic_symbol): Likewise.
	* elfxx-mips.c (_bfd_mips_elf_adjust_dynamic_symbol): Likewise.
	* elfxx-sparc.c (_bfd_sparc_elf_gc_mark_hook): Likewise.
	(_bfd_sparc_elf_adjust_dynamic_symbol): Likewise.
	* elfxx-tilegx.c (tilegx_elf_gc_mark_hook): Likewise.
	(tilegx_elf_adjust_dynamic_symbol): Likewise.
	* elfxx-x86.c (_bfd_x86_elf_adjust_dynamic_symbol): Likewise.