libcxx-3.8: apply Darwin-specific patch only on Darwin

... to better decouple different platforms (in future).

pkgs/development/compilers/llvm/3.8/libc++/default.nix

···
     cmakeFlagsArray=($cmakeFlagsArray -DLIBCXX_CXX_ABI_INCLUDE_PATHS="$NIX_BUILD_TOP/libcxxabi-${version}.src/include")
   '';
 
-  patches = [ ./darwin.patch ];
+  patches = lib.optional stdenv.isDarwin ./darwin.patch;
 
   buildInputs = [ cmake libcxxabi ] ++ lib.optional stdenv.isDarwin fixDarwinDylibNames;