Add -resource-dir flag to nix-support/cc-cflags on Darwin

This fixes a problem on Darwin when running `bazel coverage` failed because the
runtime profiling library (libclang_rt.profile_osx.a) was not found.

(see tweag/rules_haskell#1768)

Note, this sets the resource dir to the `/resource-root` directory of the
original wrapped `cc`; otherwise we would have to re-create this directory for
our own wrapper again.

toolchains/cc/cc.nix

@@ -41,6 +41,7 @@ let
         echo "-L${pkgs.llvmPackages.libcxxabi}/lib" >> $out/nix-support/cc-cflags
         echo "-L${pkgs.libiconv}/lib" >> $out/nix-support/cc-cflags
         echo "-L${pkgs.darwin.libobjc}/lib" >> $out/nix-support/cc-cflags
+        echo "-resource-dir=${pkgs.stdenv.cc}/resource-root" >> $out/nix-support/cc-cflags
       '';
     };
   cc =