Support compressed debug sections in dynamic object files.

Support compressed debug sections in dynamic object files.

This patch adds support for reading compressed debug info in
shared objects. It actually simplifies things, by moving the
support for compressed sections all the way up to the top-level
Object class, eliminating the need for several virtual methods.

gold/
	* dwp.cc (Sized_relobj_dwo::do_section_contents): Delete.
	(Sized_relobj_dwo::setup): Build compressed section map.
	(Sized_relobj_dwo::do_decompressed_section_contents): Delete.
	* dynobj.cc (Sized_dynobj::base_read_symbols): Build compressed
	section map.
	* object.cc (Sized_relobj_file::Sized_relobj_file): Remove
	compressed_sections_ field.
	(build_compressed_section_map): Take Object instead of
	Sized_relobj_file parameter; add decompress_if_needed parameter.
	(Sized_relobj_file::do_find_special_sections): Store compressed
	section map in parent Object.
	(Sized_relobj_file::do_decompressed_section_contents): Move
	implementation to Object::decompressed_section_contents.
	(Sized_relobj_file::do_discard_decompressed_sections): Move
	implementation to Object::discard_decompressed_sections.
	* object.h (build_compressed_section_map): Declare.
	(Object::Object): Add compressed_sections_ field.
	(Object::section_is_compressed): Move implementation here.
	(Object::decompressed_section_contents): De-virtualize.
	(Object::discard_decompressed_sections): De-virtualize.
	(Object::do_section_is_compressed): Delete.
	(Object::do_decompressed_section_contents): Delete.
	(Object::set_compressed_sections): New method.
	(Object::compressed_sections): New method.
	(Object::compressed_sections_): New data member.
	(Compressed_section_info, Compressed_section_map): Move to top of file.
	(Sized_relobj_file::do_section_is_compressed): Delete.
	(Sized_relobj_file::do_decompressed_section_contents): Delete.
	(Sized_relobj_file::do_discard_decompressed_sections): Delete.
	(Sized_relobj_file::compressed_sections_): Move to Object class.