Enables win64 build of yuv_convert_simd_x86 without rolling x86inc.asm forward.

BUG=172129, 166496

Review URL: https://codereview.chromium.org/12036108

Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a42caffa71b3ea2b1ac481588353caa816795e08
diff --git a/yasm_compile.gypi b/yasm_compile.gypi
index 31ce6cf..0d9263f 100644
--- a/yasm_compile.gypi
+++ b/yasm_compile.gypi
@@ -70,7 +70,7 @@
       }],
       [ 'OS=="win" and target_arch=="x64"', {
         'yasm_flags': [
-          '-fx64',
+          '-fwin64',
           '-m', 'amd64',
         ],
       }],