dwarf.exp: Allow generating a stub .debug_line section

dwarf.exp: Allow generating a stub .debug_line section

Example of use:

	Dwarf::assemble "foo.s" {
	    build_id 0102030405060708

	    declare_labels L;
	    cu {is_64 0 version 4 addr_size 8} {
		DW_TAG_compile_unit {
		    {DW_AT_stmt_list $L DW_FORM_sec_offset}
		} {
		    DW_TAG_subprogram {
			# We can now reference the source file.
			{DW_AT_decl_file 1 DW_FORM_data1}
		    }
		}
	    }

	    lines {is_64 0 version 2 addr_size 8} L {
		include_dir "foo"
		include_dir "bar"
		file_name "foo.c" 1
		file_name "bar.c" 1
		file_name "baz.c" 2
	    }
	}

Signed-off-by: Petr Machata <pmachata@redhat.com>