clang on windows support

1) Run python bin/fetch-clang-win
2) Set clang_win = "../bin/clang_win"
3) ???
4) Profit

Most changes here are to pass the right -mfoo flags to Clang
to enable advanced instruction sets, or fixed warning-as-errors.

BUG=skia:2679

Change-Id: Ieed145d35c209131c7c16fdd3ee11a3de4a1a921
Reviewed-on: https://skia-review.googlesource.com/28740
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/third_party/zlib/BUILD.gn b/third_party/zlib/BUILD.gn
index 3f9bc5c..a1e4836 100644
--- a/third_party/zlib/BUILD.gn
+++ b/third_party/zlib/BUILD.gn
@@ -48,7 +48,7 @@
       "../externals/zlib/fill_window_sse.c",
       "../externals/zlib/x86.c",
     ]
-    if (!is_win) {
+    if (!is_win || is_clang) {
       cflags_c = [
         "-msse4.2",
         "-mpclmul",