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.

pkgs/applications/virtualization/crosvm/default.nix

@@ -75,7 +75,7 @@ in
 
     CROSVM_CARGO_TEST_KERNEL_BINARY =
       lib.optionalString (stdenv.buildPlatform == stdenv.hostPlatform)
-        "${linux}/${stdenv.hostPlatform.platform.kernelTarget}";
+        "${linux}/${stdenv.hostPlatform.linux-kernel.target}";
 
     passthru = {
       inherit adhdSrc;

pkgs/applications/virtualization/vpcs/default.nix

@@ -13,7 +13,7 @@ stdenv.mkDerivation rec {
 
   buildPhase = ''(
     cd src
-    ./mk.sh ${stdenv.buildPlatform.platform.kernelArch}
+    ./mk.sh ${stdenv.buildPlatform.linuxArch}
   )'';
 
   installPhase = ''