Add support for win64 flags to yasm.

This change is needed to add win64 support to libvpx.

BUG=166496, 170672


Review URL: https://chromiumcodereview.appspot.com/11971037

Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d661f0522c34389d2a330d4b4ea12366127892fc
diff --git a/yasm_compile.gypi b/yasm_compile.gypi
index da3ef80..504abc2 100644
--- a/yasm_compile.gypi
+++ b/yasm_compile.gypi
@@ -68,6 +68,13 @@
           '-m', 'x86',
         ],
       }],
+      [ 'OS=="win" and target_arch=="x64"', {
+        'yasm_flags': [
+          '-DPREFIX',
+          '-fx64',
+          '-m', 'amd64',
+        ],
+      }],
 
       # Define output extension.
       ['OS=="win"', {