Crash due to unaligned loads when building with clang 6.0

When I built the whole tree with clang 6.0, all_types-itest crashed due to an illegal instruction. Looking at assembly, it appeared to be that clang had generated a 'movaps' (aligned load) instruction for a *reinterpret_cast<int128_t>() call loading into an xmm register. We aren't careful with alignment about loading other integer types because unaligned loads of int64s don't have a high penalty, but unaligned load of int128 causes a crash.

This is likely to crash on other compilers too – surprised we haven't seen it yet.