lib: Clean up how linux and gcc config is specified

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

This reverts commit 0bc275e634.

pkgs/applications/virtualization/crosvm/default.nix

@@ -75,7 +75,7 @@ in
 
     CROSVM_CARGO_TEST_KERNEL_BINARY =
       lib.optionalString (stdenv.buildPlatform == stdenv.hostPlatform)
-        "${linux}/${stdenv.hostPlatform.platform.kernelTarget}";
+        "${linux}/${stdenv.hostPlatform.linux-kernel.target}";
 
     passthru = {
       inherit adhdSrc;

pkgs/applications/virtualization/vpcs/default.nix

@@ -13,7 +13,7 @@ stdenv.mkDerivation rec {
 
   buildPhase = ''(
     cd src
-    ./mk.sh ${stdenv.buildPlatform.platform.kernelArch}
+    ./mk.sh ${stdenv.buildPlatform.linuxArch}
   )'';
 
   installPhase = ''