Remove reference to Solaris 9 in comment of add_options_for_tls

Since r5-172-gd9f069ab4f6450, the code no longer matches the
comment as the code for Solaris 9 support was removed.
This just updates the comment to reference AIX only as
the code does.

Committed as obvious.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp (add_options_for_tls): Remove
	reference to Solaris 9 in comments.