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/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.