Add AVX flag for Windows GN builds

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 45eecb640340188604ca1dd063415fda23df880a..0011d68558a41f834db68d5d77ae959c6caec10c 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -131,7 +131,9 @@ static_library("libvpx_intrinsics_avx2") {
   configs += [ ":libvpx_config" ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
-  if (!is_win) {
+  if (is_win) {
+    cflags = [ "/arch:AVX" ]

[Tom Finegan, 2015/01/22 18:43:49]

msdn vs2013 docs[1] say both are supported. I think vs2013 is the minimum
required compiler for windows chromium development, so AVX2 should be OK (and is
probably what you want since we're using -mavx2 everywhere else).

[1] https://msdn.microsoft.com/en-us/library/jj620901.aspx

+  } else {
     cflags = [ "-mavx2" ]
   }
   if (cpu_arch == "x86") {