Make Elf_file::section_name() a const function, so that it can be used in

Make Elf_file::section_name() a const function, so that it can be used in
places where we have only a const Elf_file*.

elfcpp/
	* elfcpp_file.h (Elf_file::shnum): New const function.
	(Elf_file::shstrndx): New const function.
	(Elf_file::large_shndx_offset): New const function.
	(Elf_file::section_name): Add const attribute.
	(Elf_file::section_header_offset): Likewise.

gold/
	* dwp.cc (Sized_relobj_dwo::do_section_name): Add const attribute.
	* dynobj.h (Sized_dynobj::do_section_name): Likewise.
	* incremental.cc (Sized_relobj_incr::do_section_name): Likewise.
	(Sized_incr_dynobj::do_section_name): Likewise.
	* incremental.h (Sized_relobj_incr::do_section_name): Likewise.
	(Sized_incr_dynobj::do_section_name): Likewise.
	* object.h (Object::section_name): Likewise.
	(Object::do_section_name): Likewise.
	(Sized_relobj_file::do_section_name): Likewise.
	* plugin.cc (Sized_pluginobj::do_section_name): Likewise.
	* plugin.h (Sized_pluginobj::do_section_name): Likewise.