haskell: add combinator to enable DWARF debugging

Tested using:

$ cat test-strip.nix
with (import ./. {});
with haskell.lib;

haskell.packages.ghc802.override {
  overrides = self: super: {
    microlens = enableDWARFDebugging super.microlens;
  };
}

$ nix-build -A microlens test-strip.nix
$ objdump -t <path-to-so-file>