Force the use of cmpxchg16b on x86_64.
Wikipedia suggests that only very old AMD64 (circa 2004) did not have
this instruction. Further, it's required by Windows 8 so no new cpus
will ever omit it.
If we truely care about these, then we could check this at startup time
and then avoid executing paths that use it.
Backports commit 7ebee43ee3e2fcd7b5063058b7ef74bc43216733 from qemu
Allows Int128 to be used more generally, rather than having to
begin with 64-bit inputs and accumulate.
Backports commit 1edaeee0955fba7d834b7c8f4e372e7eae030745 from qemu
