Fix oddity in write_gdbindex

Fix oddity in write_gdbindex

My recent patch to unify CUs and TUs introduced an oddity in
write_gdbindex.  Here, we pass 'i' to recursively_write_psymbols, but
we must instead pass 'counter', to handle the situation where a TU is
mixed in with the CUs.

I am not sure a test case for this is possible.  I think it can only
happen when using DWARF 5, where a TU appears in .debug_info.
However, this situation is already not handled correctly by
.gdb_index.  I filed a bug about this.