[libcxxabi] Fix alignment of allocated exceptions in 32 bit builds

Summary:
In 32 bit builds on a 64 bit system `std::malloc` does not return correctly aligned memory.  This leads to undefined behavior.

This patch switches to using `posix_memalign` to allocate correctly aligned memory instead.

Reviewers: mclow.lists, danalbert, jroelofs, compnerd

Reviewed By: compnerd

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25417

llvm-svn: 296952