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/development/compilers/julia/1.0.nix

@@ -123,7 +123,7 @@ stdenv.mkDerivation rec {
     let
       arch = stdenv.lib.head (stdenv.lib.splitString "-" stdenv.system);
       march = {
-        x86_64 = stdenv.hostPlatform.platform.gcc.arch or "x86-64";
+        x86_64 = stdenv.hostPlatform.gcc.arch or "x86-64";
         i686 = "pentium4";
         aarch64 = "armv8-a";
       }.${arch}

pkgs/development/compilers/julia/1.3.nix

@@ -73,7 +73,7 @@ stdenv.mkDerivation rec {
     let
       arch = head (splitString "-" stdenv.system);
       march = {
-        x86_64 = stdenv.hostPlatform.platform.gcc.arch or "x86-64";
+        x86_64 = stdenv.hostPlatform.gcc.arch or "x86-64";
         i686 = "pentium4";
         aarch64 = "armv8-a";
       }.${arch}

pkgs/development/compilers/julia/1.5.nix

@@ -71,7 +71,7 @@ stdenv.mkDerivation rec {
     let
       arch = head (splitString "-" stdenv.system);
       march = {
-        x86_64 = stdenv.hostPlatform.platform.gcc.arch or "x86-64";
+        x86_64 = stdenv.hostPlatform.gcc.arch or "x86-64";
         i686 = "pentium4";
         aarch64 = "armv8-a";
       }.${arch}