Existing "mips64el" should be "mipsel".
This is just the barest minimum so that nixpkgs can recognize them as
systems - although required for building individual derivations onto
MIPS boards, it is not sufficient if you want to actually build nixos on
those targets
The old hard-coded lists are now used to test system parsing.
In the process, make an `assertTrue` in release lib for eval tests; also
use it in release-cross
