[SystemZ] Actually enable -mzvector keywords

It appears when initially committing the support for the IBM Z vector
extension language, one critical line was lost, causing the specific
keywords __vector, __bool, and vec_step to not actually be enabled.
(Note that this does not affect "vector" and "bool"!)

Unfortunately, this was not caught by any tests either.  (All existing
Z vector tests just use the regular "vector" and "bool" keywords ...)

Fixed by adding the missing line and updating the tests.

llvm-svn: 344611