[PATCH 3/3] gold: Handle DW_AT_high_pc as offset from DW_AT_low_pc in gdb-index.cc.

[Cary Coutant]

>    * testsuite/gdb_index_test_3.c: New test source file.
>    * testsuite/gdb_index_test_3.sh: New test source file.

These two new files were accidentally omitted from the commit. I've
checked them in.

-cary