lib: Clean up how linux and gcc config is specified

The `platform` field is pointless nesting: it's just stuff that happens
to be defined together, and that should be an implementation detail.

This instead makes `linux-kernel` and `gcc` top level fields in platform
configs. They join `rustc` there [all are optional], which was put there
and not in `platform` in anticipation of a change like this.

`linux-kernel.arch` in particular also becomes `linuxArch`, to match the
other `*Arch`es.

The next step after is this to combine the *specific* machines from
`lib.systems.platforms` with `lib.systems.examples`, keeping just the
"multiplatform" ones for defaulting.

nixos/modules/services/misc/nix-daemon.nix

@@ -587,10 +587,10 @@ in
 
     nix.systemFeatures = mkDefault (
       [ "nixos-test" "benchmark" "big-parallel" "kvm" ] ++
-      optionals (pkgs.hostPlatform.platform ? gcc.arch) (
-        # a builder can run code for `platform.gcc.arch` and inferior architectures
-        [ "gccarch-${pkgs.hostPlatform.platform.gcc.arch}" ] ++
-        map (x: "gccarch-${x}") lib.systems.architectures.inferiors.${pkgs.hostPlatform.platform.gcc.arch}
+      optionals (pkgs.hostPlatform ? gcc.arch) (
+        # a builder can run code for `gcc.arch` and inferior architectures
+        [ "gccarch-${pkgs.hostPlatform.gcc.arch}" ] ++
+        map (x: "gccarch-${x}") lib.systems.architectures.inferiors.${pkgs.hostPlatform.gcc.arch}
       )
     );