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/os-specific/linux/uclibc/default.nix

@@ -68,7 +68,7 @@ stdenv.mkDerivation {
     cat << EOF | parseconfig
     ${nixConfig}
     ${extraConfig}
-    ${stdenv.hostPlatform.platform.uclibc.extraConfig or ""}
+    ${stdenv.hostPlatform.uclibc.extraConfig or ""}
     EOF
     ( set +o pipefail; yes "" | make oldconfig )
   '';