Headers: explicitly specify double-word alignment

GCC will interpret `__attribute__((__aligned__))` as 8-byte alignment on
ARM, but clang will not.  Explicitly specify the alignment.  This
mirrors the declaration in libunwind.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@311576 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed