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/inspect.nix

@@ -47,6 +47,7 @@ rec {
     isMinGW        = { kernel = kernels.windows; abi = abis.gnu; };
     isWasi         = { kernel = kernels.wasi; };
     isGhcjs        = { kernel = kernels.ghcjs; };
+    isGenode       = { kernel = kernels.genode; };
     isNone         = { kernel = kernels.none; };
 
     isAndroid      = [ { abi = abis.android; } { abi = abis.androideabi; } ];