top-level: Only get rid of target-observing overrides when cross-comp…

…iling

Previously, all overrides from the final native stage were chucked. This is
sound but caused unnecessary rebuilds. It would be tempting to just keep
all overrides, but then pkgs like gcc and binutils would not target the
correct platform--they would still target the (native) build platform. Now,
each native final stage responsibly just overrides such packages of the
target platform is that used to build the overriding packages.

pkgs/stdenv/linux/default.nix

@@ -300,10 +300,12 @@ in
 
       overrides = self: super: {
         inherit (prevStage)
-          gcc-unwrapped
-          gzip bzip2 xz bash binutils coreutils diffutils findutils gawk
+          gzip bzip2 xz bash coreutils diffutils findutils gawk
           glibc gnumake gnused gnutar gnugrep gnupatch patchelf
           attr acl paxctl zlib pcre;
+      } // lib.optionalAttrs (super.targetPlatform == localSystem) {
+        # Need to get rid of these when cross-compiling.
+        inherit (prevStage) binutils gcc-unwrapped;
       };
     };
   })