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/build-support/bintools-wrapper/default.nix

@@ -190,7 +190,7 @@ stdenv.mkDerivation {
       else if targetPlatform.isRiscV then "lriscv"
       else throw "unknown emulation for platform: ${targetPlatform.config}";
     in if targetPlatform.useLLVM or false then ""
-       else targetPlatform.platform.bfdEmulation or (fmt + sep + arch);
+       else targetPlatform.bfdEmulation or (fmt + sep + arch);
 
   strictDeps = true;
   depsTargetTargetPropagated = extraPackages;