ARM: don't try to over-align large vectors as arguments.

By default LLVM thinks very large vectors get aligned to their size when
passed across functions. Unfortunately no-one told the ARM backend so it
doesn't trigger stack realignment and so accesses can cause the usual
misalignment issues (e.g. a data abort).

This changes the ABI alignment to the stack alignment, which in practice
(and as a bonus) also coincides with the alignment "natural" vectors get.

llvm-svn: 331451