WriteHaskell: Strip binaries

This reduces the size of Hello World [1] from 3.06 MiB to 678 KiB.

[1] As measured by nix-shell -p 'writers.writeHaskellBin "hello" {} "main = putStrLn \"hello\""' --run 'ls -l `which hello`'

pkgs/build-support/writers/default.nix

@@ -148,6 +148,7 @@ rec {
         cp $contentPath tmp.hs
         ${ghc.withPackages (_: libraries )}/bin/ghc tmp.hs
         mv tmp $out
+        ${pkgs.binutils-unwrapped}/bin/strip --strip-unneeded "$out"
       '';
     } name;