toplevel: Unbreak Ada build [PR123490]

toplevel: Unbreak Ada build [PR123490]

As written earlier, the config-ml.in change from the
--with-multi-buildlist patch broke build of Ada, Ada uses
RTSDIR = rts$(subst /,_,$(MULTISUBDIR))
and expects that the primary multilib will result in rts
rather than rts_. it results in after the --with-multi-buildlist
changes.

The following patch fixes it by restoring previous behavior for
ml_subdir / MULTISUBDIR such that for primary multilib it is
still empty rather than /.

2026-01-10  Jakub Jelinek  <jakub@redhat.com>

	PR ada/123490
	* config-ml.in: Restore ml_subdir being empty instead of /.
	for the primary multilib.