pass 'win64' instead of 'x64' to the assembler

nasm does not accept x64

yasm has accepted (and appears to prefer) win64 at least as far back as
1.0.0:
http://yasm.tortall.net/releases/Release1.0.0.html

Cherry picked from libvpx 460dbc0

Change-Id: I603981be6379732f5b8d5940294a79cef4b16c89
diff --git a/build/make/configure.sh b/build/make/configure.sh
index b18173b..8ca2213 100644
--- a/build/make/configure.sh
+++ b/build/make/configure.sh
@@ -1276,7 +1276,7 @@
           EXE_SFX=.exe
           ;;
         win64)
-          add_asflags -f x64
+          add_asflags -f win64
           enabled debug && add_asflags -g cv8
           EXE_SFX=.exe
           ;;