rust: preliminary cross compilation support

Moves some things to native build inputs in rust infrastructure to get
things working. rustPlatform has to get rust from buildPackages in
order to get the proper version.

pkgs/build-support/rust/default.nix

@@ -45,7 +45,8 @@ in stdenv.mkDerivation (args // {
 
   patchRegistryDeps = ./patch-registry-deps;
 
-  buildInputs = [ cacert git rust.cargo rust.rustc ] ++ buildInputs;
+  nativeBuildInputs = [ cacert git rust.cargo rust.rustc ];
+  inherit buildInputs;
 
   patches = cargoPatches ++ patches;
 

pkgs/development/compilers/rust/rustc.nix

@@ -57,7 +57,7 @@ stdenv.mkDerivation {
                 ++ [ "--enable-local-rust" "--local-rust-root=${rustPlatform.rust.rustc}" "--enable-rpath" ]
                 ++ [ "--enable-vendor" ]
                 # ++ [ "--jemalloc-root=${jemalloc}/lib"
-                ++ [ "--default-linker=${targetPackages.stdenv.cc}/bin/cc" ]
+                ++ [ "--default-linker=${stdenv.cc}/bin/cc" ]
                 ++ optional (!forceBundledLLVM) [ "--enable-llvm-link-shared" ]
                 ++ optional (targets != []) "--target=${target}"
                 ++ optional (!forceBundledLLVM) "--llvm-root=${llvmShared}";

pkgs/top-level/all-packages.nix

@@ -7229,7 +7229,7 @@ with pkgs;
 
   defaultCrateOverrides = callPackage ../build-support/rust/default-crate-overrides.nix { };
 
-  rustPlatform = recurseIntoAttrs (makeRustPlatform rust);
+  rustPlatform = recurseIntoAttrs (makeRustPlatform buildPackages.rust);
 
   makeRustPlatform = rust: lib.fix (self:
     let