Unify the behaviour of ld.bfd and ld.gold with respect to warning about unresolved...

Unify the behaviour of ld.bfd and ld.gold with respect to warning about unresolved symbol references.  (PR 24613)

	PR binutils/24613
include	* bfdlink.h (enum report_method): Delete RM_GENERATE_WARNING and
	RM_GENERATE_ERROR. Add RM_DIAGNOSE.
	(struct bfd_link_info): Add warn_unresolved_syms.

ld	* lexsup.c (parse_args): Change RM_GENERATE_WARNING and
	RM_GENERATE_ERROR to RM_DIAGNOSE.
	* emultempl/aix.em (ld_${EMULATION_NAME}_emulation): Change
	RM_GENERATE_ERROR to RM_DIAGNOSE.
	* emultempl/elf.em (ld_${EMULATION_NAME}_emulation): Likewise.

bfd	* coff-rs6000.c (xcoff_ppc_relocate_section): Change RM_GENERATE_ERROR
	to RM_DIAGNOSE plus a check of warn_unresolved_syms.
	* coff64-rs6000.c (xcoff_ppc_relocate_section): Likewise.
	* elf-bfd.h (_bfd_elf_large_com_section): Likewise.
	* elf32-m32r.c (m32r_elf_relocate_section): Likewise.
	* elf32-score.c (s3_bfd_score_elf_relocate_section): Likewise.
	* elf32-score7.c (s7_bfd_score_elf_relocate_section): Likewise.
	* elf32-sh.c (sh_elf_relocate_section): Likewise.
	* elf32-spu.c (spu_elf_relocate_section): Likewise.
	* elf64-hppa.c (elf64_hppa_relocate_section): Likewise.
	* elflink.c (elf_link_output_extsym): Likewise.
	* elfxx-mips.c (mips_elf_calculate_relocation): Likewise.