configure: disable cpunop if the check fails

Moving cpunop from the HAVE_LIST to the ARCH_EXT_LIST_X86 has the side
effect of enabling it. The semantics of the check have to be changed
from enable if successful to disable if unsuccessful. This was missing
in 2b0bb699 causing build errors with
nasm.

configure

@@ -3758,7 +3758,7 @@ EOF
         check_yasm "movbe ecx, [5]" && enable yasm ||
             die "yasm/nasm not found or too old. Use --disable-yasm for a crippled build."
         check_yasm "vfmaddps ymm0, ymm1, ymm2, ymm3" || disable fma4_external
-        check_yasm "CPU amdnop" && enable cpunop
+        check_yasm "CPU amdnop" || disable cpunop
     fi
 
     case "$cpu" in