GCC 4.0.x on s390{,x} aligned vector_size 1/2/4/8/64/128/... types
to their size, just vector_size 16 and 32 has been 8 bytes aligned
(BIGGEST_ALIGNMENT).
It is just a side-effect of the 3.4/4.0 code - if there was a supported
integer mode on the target for the requested size, then alignment of that
mode was used (and mode alignments are at most BIGGEST_ALIGNMENT).
If no integer mode was supported for that size, it would use the earlier
alignment, i.e. vector_size. [...]
We can document the incompatibility as a feature (after all, getting
rid of that alignment anomaly on s390{,x} wouldn't be a bad thing I guess).