ghcjs-ng: fixup system tuple

on i686-linux, the libexec dir should be /libexec/i386-linux-ghc-8.2.2 not /libexec/i686-linux-ghc-8.2.2

pkgs/development/compilers/ghcjs-ng/default.nix

···
   };
 
   bootGhcjs = haskellLib.justStaticExecutables passthru.bootPkgs.ghcjs;
-  libexec = "${bootGhcjs}/libexec/${builtins.replaceStrings ["darwin"] ["osx"] stdenv.system}-${passthru.bootPkgs.ghc.name}/${bootGhcjs.name}";
+  libexec = "${bootGhcjs}/libexec/${builtins.replaceStrings ["darwin" "i686"] ["osx" "i386"] stdenv.system}-${passthru.bootPkgs.ghc.name}/${bootGhcjs.name}";
 
 in stdenv.mkDerivation {
     name = bootGhcjs.name;