* output.h (class Output_data): Add const version of
output_section and do_output_section.
(class Output_section_data): Add const version of
do_output_section.
(class Output_section): Likewise.
* layout.cc (Layout::add_target_dynamic_tags): New function.
* layout.h (class Layout): Update declarations.
* arm.cc (Target_arm::do_finalize_sections): Use
add_target_dynamic_tags.
* i386.cc (Target_i386::do_finalize_sections): Likewise.
* powerpc.cc (Target_powerpc::do_finalize_sections): Likewise.
* sparc.cc (Target_sparc::do_finalize_sections): Likewise.
* x86_64.cc (Target_x86_64::do_finalize_sections): Likewise.