gold: Handle DW_AT_high_pc as offset from DW_AT_low_pc in gdb-index.cc.

gold: Handle DW_AT_high_pc as offset from DW_AT_low_pc in gdb-index.cc.

        * dwarf_reader.cc (Dwarf_die::address_attribute): New function.
        * dwarf_reader.h (Dwarf_die::address_attribute): Likewise.
        * gdb-index.cc (Gdb_index_info_reader::record_cu_ranges): Handle
        DW_AT_high_pc as offset from DW_AT_low_pc.

        * testsuite/Makefile.am (gdb_index_test_3.sh): New test case.
        * testsuite/Makefile.in: Regenerate.
        * testsuite/gdb_index_test_3.c: New test source file.
        * testsuite/gdb_index_test_3.sh: New test source file.