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/.gitignore b/.gitignore
index 0456db8..7705bdf 100644
--- a/.gitignore
+++ b/.gitignore
@@ -43,6 +43,7 @@
 bin/gn.exe
 bin/clang-format
 bin/clang-format.exe
+bin/clang_win
 
 vulkan-out
-gl-out
\ No newline at end of file
+gl-out