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 2b0bb69997c2416e74f41aa1400ce983bf8775c0 causing build errors with
nasm.
