* options.h (class General_options): Define build_id option.

	* options.h (class General_options): Define build_id option.
	* layout.h (class Layout): Declare write_build_id, create_note,
	create_build_id.  Add build_id_note_ member.
	* layout.cc: Include <cerrno>, <fcntl.h>, <unistd.h>,
	"libiberty.h", "md5.h", "sha1.h".
	(Layout::Layout): Initialize eh_frame_data_,
	eh_frame_hdr_section_, and build_id_note_.
	(Layout::finalize): Call create_build_id.
	(Layout::create_note): New function, broken out of
	Layout::create_gold_note.
	(Layout::create_gold_note): Call create_note.
	(Layout::create_build_id): New function.
	(Layout::write_build_id): New function.
	(Close_task_runner::run): Call write_build_id.