jvmci8: restrict meta.platforms according to graalvm8

This unbreaks evaluation on darwin, maybe elsewhere.  Thread:
https://github.com/NixOS/nixpkgs/commit/3cf07efa30a#r33500279

pkgs/development/compilers/graalvm/default.nix

@@ -270,7 +270,7 @@ in rec {
     '';
     dontFixup = true; # do not nuke path of ffmpeg etc
     dontStrip = true; # why? see in oraclejdk derivation
-    inherit (openjdk) meta;
+    meta = openjdk.meta // { inherit (graalvm8.meta) platforms; };
     inherit (openjdk) postFixup;
   };