configure: Fix miscompilation for i586

If the CPU is 386, 486 or pentium, we must not use cmov in inline
assembler.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -3842,6 +3842,7 @@ elif enabled x86; then
     case $cpu in
         i[345]86|pentium)
             cpuflags="-march=$cpu"
+            disable i686
             disable mmx
         ;;
         # targets that do NOT support nopl and conditional mov (cmov)