* output.h (class Output_data): Add const version of

	* 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.