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

Change-Id: Ied881b1df0570da256b1bd7e131e7817e47f768f

build/make/configure.sh

@@ -1345,7 +1345,7 @@ EOF
           EXE_SFX=.exe
           ;;
         win64)
-          add_asflags -f x64
+          add_asflags -f win64
           enabled debug && add_asflags -g cv8
           EXE_SFX=.exe
           ;;