Fix gdb.info build failure

    gdb.texinfo:36367: `Installed System-wide Configuration Scripts' has no Up field (perhaps incorrect sectioning?).
    gdb.texinfo:36367: warning: unreferenced node `Installed System-wide Configuration Scripts'.

gdb/doc/ChangeLog:

        * gdb.texinfo (System-wide Configuration Scripts): Renames
        "Installed System-wide Configuration Scripts". Add associated
        @menu block.