gold: Add --pack-dyn-relocs=relr for arm/aarch64/x86-64 [PR 28601]

gold: Add --pack-dyn-relocs=relr for arm/aarch64/x86-64 [PR 28601]

elfcpp/
    * elfcpp.h (SHT): New constant SHT_RELR.
    (DT): New constants DT_SYMTAB_SHNDX/DT_RELRSZ/DT_RELR/DT_RELRENT.
    (Elf_sizes): New static data member relr_size.
gold/
    * options.h (class General_options): Add pack_dyn_relocs,
    Pack_dyn_relocs, relr_relocs, and pack_dyn_relocs_enum_.
    * options.cc (General_options::relr_relocs): Define.
    * reloc-types.h (Reloc_types<elfcpp::SHT_RELR, size, big_endian>):
    New.
    * layout.h (Layout::add_target_dynamic_tags): Add dyn_relr.
    * layout.cc (Layout::add_target_dynamic_tags): Add dyn_relr.
    * output.h (set_current_data_size_for_child): Remove assert.
    (Output_reloc<elfcpp::SHT_RELR, dynamic, size, big_endian>): New.
    (Output_data_reloc<elfcpp::SHT_RELR, dynamic, size, big_endian>): New.
    * output.cc (Output_data_reloc_base): Set entsize for SHT_RELR.
    (do_write): Define.
    (Output_data_reloc<elfcpp::SHT_RELR, true, *, *>): Instantiate.
    * aarch64.cc (Target_aarch64): Add Relr_section,
    relr_dyn_section, and relr_dyn_.
    (do_relax): Check SHT_RELR.
    (Target_aarch64::Scan::local): Check SHT_RELR.
    (Target_aarch64::Scan::global): Check SHT_RELR.
    * arm.cc (Target_arm): Add Relr_section,
    relr_dyn_section, and relr_dyn_.
    (do_relax): Check SHT_RELR.
    (Target_arm::Scan::local): Check SHT_RELR.
    (Target_arm::Scan::global): Check SHT_RELR.
    * x86_64.cc (Target_x86_64): Add Relr_section,
    relr_dyn_section, and relr_dyn_.
    (do_relax): Check SHT_RELR.
    (Target_x86_64::Scan::local): Check SHT_RELR.
    (Target_x86_64::Scan::global): Check SHT_RELR.
    * testsuite/Makefile.am: New tests relr_arm.sh relr_aarch64.sh
      relr_x86_64.sh.
    * testsuite/relr_1.s: New.