PR 5990

	PR 5990
	* descriptors.cc: New file.
	* descriptors.h: New file.
	* gold-threads.h (class Hold_optional_lock): New class.
	* fileread.cc: Include "descriptors.h".
	(File_read::~File_read): Release descriptor rather than closing
	it.
	(File_read::open) [file]: Call open_descriptor rather than open.
	Set is_descriptor_opened_.
	(File_read::open) [memory]: Assert that descriptor is not open.
	(File_read::reopen_descriptor): New function.
	(File_read::release): Release descriptor.
	(File_read::do_read): Make non-const.  Reopen descriptor.
	(File_read::read): Make non-const.
	(File_read::make_view): Reopen descriptor.
	(File_read::do_readv): Likewise.
	* fileread.h (class File_read): Add is_descriptor_opened_ field.
	Update declarations.
	* layout.cc: Include "descriptors.h".
	(Layout::create_build_id): Use open_descriptor rather than open.
	* output.cc: Include "descriptors.h".
	(Output_file::open): Use open_descriptor rather than open.
	* archive.cc (Archive::const_iterator): Change Archive to be
	non-const.
	(Archive::begin, Archive::end): Make non-const.
	(Archive::count_members): Likewise.
	* archive.h (class Archive): Update declarations.
	* object.h (Object::read): Make non-const.
	* Makefile.am (CCFILES): Add descriptors.cc.
	(HFILES): Add descriptors.h.
	* Makefile.in: Rebuild.