Someone once submitted this to the Debian bugtracker, but this probably should be resolved upstream. OpenBSD, the guys who came up with arc4random uses ChaCha20 since 5.5 released in May 2014
Created attachment 112357 [details] [review]
patch to add chacha20 support, taken from https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=747671#10
Taken from https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=747671#10
This is fixed now in the pu/arc4random branch at http://git.hadrons.org/cgit/libbsd.git/, which still needs some testing and some polishing, but it will be included in the upcoming release which should happen within the coming couple of weeks at most.
This has now been released as part of libbsd 0.8.0.