ghc: compile with DWARF support by default

This allows our GHCs to build programs with DWARF debug information
when -g is passed, see https://ghc.haskell.org/trac/ghc/wiki/DWARF.

Compiling with debug symbols is off by default until GHC ticket #15960
(Using -g causes differences in generated core) is fixed.

pkgs/development/compilers/ghc/head.nix

@@ -7,6 +7,8 @@
 
 , libiconv ? null, ncurses
 
+, enableDwarf ? !stdenv.isDarwin, elfutils # for DWARF support
+
 , useLLVM ? !stdenv.targetPlatform.isx86
 , # LLVM is conceptually a run-time-only depedendency, but for
   # non-x86, we need LLVM to bootstrap later stages, so it becomes a
@@ -73,7 +75,8 @@ let
   # Splicer will pull out correct variations
   libDeps = platform: stdenv.lib.optional enableTerminfo [ ncurses ]
     ++ stdenv.lib.optional (!enableIntegerSimple) gmp
-    ++ stdenv.lib.optional (platform.libc != "glibc" && !targetPlatform.isWindows) libiconv;
+    ++ stdenv.lib.optional (platform.libc != "glibc" && !targetPlatform.isWindows) libiconv
+    ++ stdenv.lib.optional enableDwarf elfutils;
 
   toolsForTarget = [
     pkgsBuildTarget.targetPackages.stdenv.cc
@@ -176,6 +179,8 @@ stdenv.mkDerivation (rec {
     "CONF_GCC_LINKER_OPTS_STAGE2=-fuse-ld=gold"
   ] ++ stdenv.lib.optionals (disableLargeAddressSpace) [
     "--disable-large-address-space"
+  ] ++ stdenv.lib.optional enableDwarf [
+    "--enable-dwarf-unwind"
   ];
 
   # Make sure we never relax`$PATH` and hooks support for compatability.