x86inc: Check for __OUTPUT_FORMAT__ having a value of "x64"

This is also a valid value for WIN64.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavutil/x86/x86inc.asm

@@ -49,6 +49,8 @@
         %define WIN64  1
     %elifidn __OUTPUT_FORMAT__,win64
         %define WIN64  1
+    %elifidn __OUTPUT_FORMAT__,x64
+        %define WIN64  1
     %else
         %define UNIX64 1
     %endif