Add support for cross compiling to js-ghcjs

This platform doesn't have a C compiler, and so relies and the changes
in the previous commit to work.

lib/systems/doubles.nix

@@ -27,6 +27,8 @@ let
     "riscv32-linux" "riscv64-linux"
 
     "aarch64-none" "avr-none" "arm-none" "i686-none" "x86_64-none" "powerpc-none" "msp430-none" "riscv64-none" "riscv32-none"
+
+    "js-ghcjs"
   ];
 
   allParsed = map parse.mkSystemFromString all;

pkgs/stdenv/booter.nix

@@ -121,9 +121,12 @@ stageFuns: let
   postStage = buildPackages: {
     __raw = true;
     stdenv.cc =
-      if buildPackages.stdenv.cc.isClang or false
+      if buildPackages.stdenv.hasCC
-      then buildPackages.clang
+      then
-      else buildPackages.gcc;
+        if buildPackages.stdenv.cc.isClang or false
+        then buildPackages.clang
+        else buildPackages.gcc
+      else buildPackages.stdenv.cc;
   };
 
 in dfold folder postStage (_: {}) withAllowCustomOverrides

pkgs/stdenv/cross/default.nix

@@ -51,12 +51,18 @@ in lib.init bootStages ++ [
       extraBuildInputs = [ ]; # Old ones run on wrong platform
       allowedRequisites = null;
 
+      hasCC = !targetPlatform.isGhcjs;
+
       cc = if crossSystem.useiOSPrebuilt or false
              then buildPackages.darwin.iosSdkPkgs.clang
            else if crossSystem.useAndroidPrebuilt or false
              then buildPackages."androidndkPkgs_${crossSystem.ndkVer}".clang
            else if targetPlatform.isGhcjs
-             then null
+             # Need to use `throw` so tryEval for splicing works, ugh.  Using
+             # `null` or skipping the attribute would cause an eval failure
+             # `tryEval` wouldn't catch, wrecking accessing previous stages
+             # when there is a C compiler and everything should be fine.
+             then throw "no C compile provided for this platform"
            else if crossSystem.useLLVM or false
              then buildPackages.llvmPackages_8.lldClang
            else buildPackages.gcc;