or1k: Fix ignored _dl_fini call

The _dl_fini function address was passed in r9 to _start but was ignored
and NULL was passed instead to __libc_start_main for the rtld_fini
argument. This patch updates the comments to explain what we are doing
and actually uses r9 for the argument.

 * ports/sysdeps/or1k/dl-start.S: Update comment.
 * ports/sysdeps/or1k/start.S: Use r9 for rtld_fini.