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/klibc/default.nix

@@ -25,11 +25,11 @@ stdenv.mkDerivation rec {
   hardeningDisable = [ "format" "stackprotector" ];
 
   makeFlags = commonMakeFlags ++ [
-    "KLIBCARCH=${stdenv.hostPlatform.platform.kernelArch}"
+    "KLIBCARCH=${stdenv.hostPlatform.linuxArch}"
     "KLIBCKERNELSRC=${linuxHeaders}"
   ] # TODO(@Ericson2314): We now can get the ABI from
     # `stdenv.hostPlatform.parsed.abi`, is this still a good idea?
-    ++ lib.optional (stdenv.hostPlatform.platform.kernelArch == "arm") "CONFIG_AEABI=y"
+    ++ lib.optional (stdenv.hostPlatform.linuxArch == "arm") "CONFIG_AEABI=y"
     ++ lib.optional (stdenv.hostPlatform != stdenv.buildPlatform) "CROSS_COMPILE=${stdenv.cc.targetPrefix}";
 
   # Install static binaries as well.