IRgen/NeXT: Fix several alignment issues with Objective-C metadata symbols:

 - Fix some places that had the alignment hard coded.
 - Use ABI type alignment, not preferred type alignment -- neither of this is exactly right, as we really want the C type alignment as required by the runtime, but the ABI alignment is a more correct choice.

This should be equivalent for x86_64, but fixes the alignment for ARM.

llvm-svn: 102314