Adding atomic primops

[Ryan Newton]

>
> For Johan's primops to work, each primop must represent a full memory
> fence that is respected both by the architecture, and by *both* compilers
> (GHC & LLVM).  Since I don't think GHC is a problem, let's talk about LLVM.
>  We need to verify that LLVM understands not to float regular loads and
> stores past one of its own atomic instructions.  If that is the case (even
> without anything being marked "volatile"), then I think we are in ok shape,
> right?
>

Clarification -- this is assuming we're using the "SequentiallyConsistent"
setting in the LLVM backend to get full fences on each op, which correspond
to the gcc-compatible __sync_* builtins:

   http://llvm.org/docs/Atomics.html#sequentiallyconsistent
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