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/installer/netboot/netboot.nix

@@ -88,7 +88,7 @@ with lib;
 
     system.build.netbootIpxeScript = pkgs.writeTextDir "netboot.ipxe" ''
       #!ipxe
-      kernel ${pkgs.stdenv.hostPlatform.platform.kernelTarget} init=${config.system.build.toplevel}/init initrd=initrd ${toString config.boot.kernelParams}
+      kernel ${pkgs.stdenv.hostPlatform.linux-kernel.target} init=${config.system.build.toplevel}/init initrd=initrd ${toString config.boot.kernelParams}
       initrd initrd
       boot
     '';