rustc: add host platform to --target when building cross-compiler

As of 1.48, std is only built for platforms in --target. If the host platform is
not included, the resulting rustc can't compile build.rs scripts.

pkgs/development/compilers/rust/rustc.nix

@@ -11,7 +11,7 @@
 }:
 
 let
- inherit (stdenv.lib) optionals optional optionalString;
+ inherit (stdenv.lib) optionals optional optionalString concatStringsSep;
  inherit (darwin.apple_sdk.frameworks) Security;
 
  llvmSharedForBuild = pkgsBuildBuild.llvm_10.override { enableSharedLibraries = true; };
@@ -72,7 +72,14 @@ in stdenv.mkDerivation rec {
  "--enable-vendor"
  "--build=${rust.toRustTargetSpec stdenv.buildPlatform}"
  "--host=${rust.toRustTargetSpec stdenv.hostPlatform}"
- "--target=${rust.toRustTargetSpec stdenv.targetPlatform}"
+ # std is built for all platforms in --target. When building a cross-compiler
+ # we need to add the host platform as well so rustc can compile build.rs
+ # scripts.
+ "--target=${concatStringsSep "," ([
+ (rust.toRustTargetSpec stdenv.targetPlatform)
+ ] ++ optionals (stdenv.hostPlatform != stdenv.targetPlatform) [
+ (rust.toRustTargetSpec stdenv.hostPlatform)
+ ])}"
 
  "${setBuild}.cc=${ccForBuild}"
  "${setHost}.cc=${ccForHost}"