lib/systems: Add Genode platform definitions

Add platform definitions for 64-bit ARM and x86. This is sufficient for
for building Genode where a toolchain is provided as an overlay.

Toolchain: git+https://git.sr.ht/~ehmry/genodepkgs?rev=14fc773ac9ecd2cbb30cb4612b284eee83d83546

lib/systems/doubles.nix

@@ -35,6 +35,8 @@ let
     "vc4-none"
 
     "js-ghcjs"
+
+    "aarch64-genode" "x86_64-genode"
   ];
 
   allParsed = map parse.mkSystemFromString all;
@@ -68,6 +70,7 @@ in {
   unix    = filterDoubles predicates.isUnix;
   wasi    = filterDoubles predicates.isWasi;
   windows = filterDoubles predicates.isWindows;
+  genode  = filterDoubles predicates.isGenode;
 
   embedded = filterDoubles predicates.isNone;