[libcxxabi] Fix alignment of pointers returned by fallback_malloc

[libcxxabi] Fix alignment of pointers returned by fallback_malloc

This aligns the ``heap[]`` array in ``fallback_malloc.cpp`` to ensure
that it can be safely cast to ``heap_node*``, and also adjusts the
allocation algorithm to ensure that every allocated block has the
alignment requested by ``__attribute__((aligned))``, by putting the
block's ``heap_node`` header 4 bytes before an aligned address.

Patch originally by Eric Fiselier: this is an updated version of
D12669, which was never landed.

Reviewed By: ldionne, #libc_abi

Differential Revision: https://reviews.llvm.org/D129842