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/parse.nix

@@ -279,6 +279,7 @@ rec {
     wasi    = { execFormat = wasm;    families = { }; };
     windows = { execFormat = pe;      families = { }; };
     ghcjs   = { execFormat = unknown; families = { }; };
+    genode  = { execFormat = elf;     families = { }; };
   } // { # aliases
     # 'darwin' is the kernel for all of them. We choose macOS by default.
     darwin = kernels.macos;
@@ -395,6 +396,8 @@ rec {
         then { cpu = elemAt l 0; vendor = "unknown"; kernel = elemAt l 1; abi = elemAt l 2; }
       else if (elemAt l 2 == "ghcjs")
         then { cpu = elemAt l 0; vendor = "unknown"; kernel = elemAt l 2; }
+      else if hasPrefix "genode" (elemAt l 2)
+        then { cpu = elemAt l 0; vendor = elemAt l 1; kernel = elemAt l 2; }
       else throw "Target specification with 3 components is ambiguous";
     "4" =    { cpu = elemAt l 0; vendor = elemAt l 1; kernel = elemAt l 2; abi = elemAt l 3; };
   }.${toString (length l)}