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/exfat/default.nix

@@ -26,7 +26,7 @@ stdenv.mkDerivation rec {
 
   makeFlags = [
     "KDIR=${kernel.dev}/lib/modules/${kernel.modDirVersion}/build"
-    "ARCH=${stdenv.hostPlatform.platform.kernelArch}"
+    "ARCH=${stdenv.hostPlatform.linuxArch}"
   ] ++ lib.optional (stdenv.hostPlatform != stdenv.buildPlatform) [
     "CROSS_COMPILE=${stdenv.cc.targetPrefix}"
   ];