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.

lib/systems/default.nix

@@ -24,8 +24,6 @@ rec {
       # Either of these can be losslessly-extracted from `parsed` iff parsing succeeds.
       system = parse.doubleFromSystem final.parsed;
       config = parse.tripleFromSystem final.parsed;
-      # Just a guess, based on `system`
-      platform = platforms.select final;
       # Determine whether we are compatible with the provided CPU
       isCompatible = platform: parse.isCompatible final.parsed.cpu platform.parsed.cpu;
       # Derived meta-data
@@ -79,7 +77,16 @@ rec {
       };
       isStatic = final.isWasm || final.isRedox;
 
-      kernelArch =
+      # Just a guess, based on `system`
+      inherit
+        ({
+          linux-kernel = args.linux-kernel or {};
+          gcc = args.gcc or {};
+          rustc = args.rust or {};
+        } // platforms.select final)
+        linux-kernel gcc rustc;
+
+      linuxArch =
         if final.isAarch32 then "arm"
         else if final.isAarch64 then "arm64"
         else if final.isx86_32 then "x86"