avrlibc: set platforms to avr-none

It will only build on avr architectures.

pkgs/development/misc/avr/libc/default.nix

@@ -25,7 +25,7 @@ stdenv.mkDerivation {
     description = "a C runtime library for AVR microcontrollers";
     homepage = http://savannah.nongnu.org/projects/avr-libc/;
     license = licenses.bsd3;
-    platforms = platforms.all;
+    platforms = [ "avr-none" ];
     maintainers = with maintainers; [ mguentner ];
   };
 }