ghcjs-*: Expose the version of GHC used

This is more meaningful than the GHCJS version for most tasks.

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

···
     targetPrefix = "";
     inherit bootGhcjs;
     inherit (bootGhcjs) version;
+    ghcVersion = bootPkgs.ghc.version;
     isGhcjs = true;
 
     enableShared = true;

pkgs/development/compilers/ghcjs/base.nix

···
   '';
   passthru = {
     inherit bootPkgs;
+    ghcVersion = ghc.version;
     isCross = true;
     isGhcjs = true;
     inherit nodejs ghcjsBoot;