x86inc: support yasm -f win64 flag also.

This sets __OUTPUT_FORMAT__ to win64 instead of win32, even though both
(through -m amd64) produce 64-bit binary code.
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavutil/x86/x86inc.asm

@@ -40,6 +40,8 @@
 %if ARCH_X86_64
     %ifidn __OUTPUT_FORMAT__,win32
         %define WIN64  1
+    %elifidn __OUTPUT_FORMAT__,win64
+        %define WIN64  1
     %else
         %define UNIX64 1
     %endif