lib: Clean up how linux and gcc config is specified

Second attempt of 8929989614589ee3acd070a6409b2b9700c92d65; see that
commit for details.

This reverts commit 0bc275e634.

pkgs/os-specific/linux/rtl8723bs/default.nix

@@ -16,7 +16,7 @@ stdenv.mkDerivation rec {
   buildInputs = [ nukeReferences ];
 
   makeFlags = [
-    "ARCH=${stdenv.hostPlatform.platform.kernelArch}" # Normally not needed, but the Makefile sets ARCH in a broken way.
+    "ARCH=${stdenv.hostPlatform.linuxArch}" # Normally not needed, but the Makefile sets ARCH in a broken way.
     "KSRC=${kernel.dev}/lib/modules/${kernel.modDirVersion}/build" # Makefile uses $(uname -r); breaks us.
   ];