[SystemZ] Improve handling of vector alignments.

Make the DataLayout string always hold a vector alignment of 8 bytes,
regardless of the vector ABI. This makes the datalayout depend only on the
target triple which is the general expectation (in assertions).

On older architectures where vectors use the natural alignment (16 bytes),
the front end will maintain the same behavior and produce an overalignment
compared to the datalayout.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D131158