lib, treewide: Add missing MIPS arches, and fix existing usage

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

lib/systems/examples.nix

@@ -51,7 +51,7 @@ rec {
   };
 
   fuloongminipc = rec {
-    config = "mips64el-unknown-linux-gnu";
+    config = "mipsel-unknown-linux-gnu";
     arch = "mips";
     float = "hard";
     platform = platforms.fuloong2f_n32;