Don't add DW_AT_low_pc if the CU has no associated code.

Don't add DW_AT_low_pc if the CU has no associated code.

        * dwarf2out.c (dwarf2out_finish): Don't add low_pc and/or
        high_pc attribute if the CU has no associated code. Only output
        DW_AT_entry_pc for CU if not generating strict dwarf and
        dwarf_version < 4.

From-SVN: r171846