avrlibc: enable parallel builds

this cuts about 80% off the build time with no apparent negative effects.

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

@@ -15,6 +15,8 @@ stdenv.mkDerivation rec {
   stripDebugList = [ "bin" ];
   dontPatchELF = true;
 
+  enableParallelBuilding = true;
+
   passthru = {
     incdir = "/avr/include";
   };