[SystemZ] Add support for IBM z14 processor (2/3)

This patch extends the -fzvector language feature to enable the new
"vector float" data type when compiling at -march=z14.  This matches
the updated extension definition implemented by other compilers for
the platform, which is indicated to applications by pre-defining
__VEC__ to 10302 (instead of 10301).

llvm-svn: 308198