x86: respect ".arch nonop" when selecting which NOPs to emit

Making GENERIC64 a special case was never correct; prior to the
generalization of ".arch .no*" to cover all ISA extensions other
processor families supporting long NOPs should have been covered as
well. When introducing ".arch .nonops" (among others) it wasn't
apparent that a hidden implication of .cpunop not being possible to
separately turn off existed here. Seeing that the two large case label
blocks in the 2nd switch() already had identical behavior, simply
collapse all of the (useful) case labels into a single "default" one.