On APCS, only try to align aggregates to 32 bits instead of 64.

This matches clang's behavior and since it is only a preference, it is not
an ABI issue.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197526 91177308-0d34-0410-b5e6-96231b3b80d8