Bug 1481513 - disable ffvpx on aarch64 windows; r=gps

There's no support for it in our local tree.

old-configure.in

@@ -2933,6 +2933,11 @@ case "$CPU_ARCH" in
       fi
   ;;
   arm*|aarch64)
+      case "$OS_TARGET" in
+        WINNT)
+          dnl don't set up anything special
+          ;;
+        *)
           MOZ_FFVPX=1
           MOZ_FFVPX_FLACONLY=1
           dnl Use same conditional as MOZ_LIBVPX to enable FFmpeg's ffvpx assembly decoder.
@@ -2940,6 +2945,8 @@ case "$CPU_ARCH" in
           FFVPX_ASFLAGS=$VPX_ASFLAGS
           ;;
       esac
+  ;;
+esac
 
 if test -n "$MOZ_FFVPX"; then
   AC_DEFINE(MOZ_FFVPX)