I cannot point you in the direction of LIBRE, but there is another San Serif that is modestly similar to Rotis 55 and also designed by Southern Software.
It is called Minima SSK, and you can find it for free by going to - http://www.abstractfonts.com/font/218
Did not bookmark any fonts. To bookmark a font, open the details for any of the fonts on AF and flip the Bookmarked status from "no" to "yes"
