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/system/activation/top-level.nix

@@ -190,7 +190,7 @@ in
 
     system.boot.loader.kernelFile = mkOption {
       internal = true;
-      default = pkgs.stdenv.hostPlatform.platform.kernelTarget;
+      default = pkgs.stdenv.hostPlatform.linux-kernel.target;
       type = types.str;
       description = ''
         Name of the kernel file to be passed to the bootloader.