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

pkgs/stdenv/adapters.nix

@@ -94,7 +94,9 @@ rec {
               # without proper `file` command, libtool sometimes fails
               # to recognize 64-bit DLLs
             ++ stdenv.lib.optional (hostPlatform.config == "x86_64-w64-mingw32") pkgs.file
-            ++ stdenv.lib.optional (hostPlatform.isAarch64 || hostPlatform.libc == "musl") pkgs.updateAutotoolsGnuConfigScriptsHook
+            ++ stdenv.lib.optional
+                 (hostPlatform.isAarch64 || hostPlatform.isMips || hostPlatform.libc == "musl")
+                 pkgs.updateAutotoolsGnuConfigScriptsHook
             ;
 
           crossConfig = hostPlatform.config;