R_MMIX_BASE_PLUS_OFFSET relocs.
* elf64-mmix.c (struct bpo_reloc_section_info, struct
bpo_reloc_request, struct bpo_greg_section_info): New.
(mmix_elf_check_common_relocs, mmix_elf_gc_sweep_hook,
bpo_reloc_request_sort_fn, mmix_elf_relax_section,
_bfd_mmix_check_all_relocs,
_bfd_mmix_prepare_linker_allocated_gregs,
_bfd_mmix_finalize_linker_allocated_gregs): New functions.
(elf_mmix_howto_table): Correct src_mask for most relocs.
(mmix_elf_perform_relocation) <case R_MMIX_BASE_PLUS_OFFSET>: New
case.
(mmix_final_link_relocate) <case R_MMIX_BASE_PLUS_OFFSET>: New
case. Fix typo in comment. New label do_mmix_reloc.
(mmix_elf_check_relocs): Abuse bfd_link_info member base_file to
store first object file with a base-plus-offset reloc. Call
mmix_elf_check_common_relocs for the part common with mmo.
(mmix_elf_final_link): Write out linker-allocated register
contents section.
(elf_backend_gc_sweep_hook): Define.
(bfd_elf64_bfd_relax_section): Define.
* mmo.c: Don't include <ctype.h>
(mmo_init): Correct init-once logic.