configure: x86: Only enable cpunop on i686

The assembler may insert nopl instructions for cpunop, which are not
(universally) supported on i586 CPUs.

configure

@@ -3431,7 +3431,7 @@ EOF
         check_yasm "vextractf128 xmm0, ymm0, 0" && enable yasm ||
             die "yasm not found, 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" && enabled i686 && enable cpunop
     fi
 
     case "$cpu" in