[SystemZ] Ensure -mno-vx disables any use of vector features

When passing the -vector feature to LLVM (or equivalently the
-mno-vx command line argument to clang), the intent is that
generated code must not use any vector features (in particular,
no vector registers must be used).

However, there are some cases where we still could generate
such uses; these are all related to some of the additional
vector features (like +vector-enhancements-1).  Since none
of those features are actually usable with -vector, just make
sure we disable them all if -vector is given.