[SystemZ] Fix handling of vectors and their exposure of the vector ABI.

- Global vector variables expose the vector ABI through their alignments only
  if they are >=16 bytes in size.

- Vectors passed between functions expose the vector ABI only if they are
  <=16 bytes in size.

LLVM test suite builds with gcc/clang now give the same gnu attributes emitted.

Reviewed By: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D141409