Fix incorrectly aligned exceptions in 32 bit builds.

This patch fixes a bug where exceptions in 32 bit builds
would be incorrectly aligned because malloc only provides 8 byte aligned
memory where 16 byte alignment is needed.

This patch makes libc++abi correctly use posix_memalign when it's
available. This requires defining _LIBCPP_BUILDING_LIBRARY so that
libc++ only defines _LIBCPP_HAS_NO_ALIGNED_ALLOCATION when libc doesn't
support it and not when aligned new/delete are disable for other
reasons.

This bug somehow made it into the 7.0 release, making it a regression.
Therefore this patch should be included in the next dot release.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@342815 91177308-0d34-0410-b5e6-96231b3b80d8