Stabilize cmpxchg16b

[sfackler]

Is anything holding up stabilization of double-wide CAS? It doesn't seem so from some searching.

[gnzlbg]

Weird, I thought I saw a stabilization PR for this one somewhere. Stabilizing this sounds good to me.

[thomcc]

Is there some story here? I've always assumed that it was intentionally left unstabilized in order to avoid stabilizing 128 bit atomics in some form prior to Atomic[UI]128.

However, with asm! stabilization on the way, this ship will sail, so I don't a reason not to.

[Amanieu]

I'm happy to accept a PR for stabilizing it. I only have one minor nit about the implementation: the ud2 should be changed to a normal panic.