Adding atomic primops

[Edward Z. Yang]

Hey Johan,

Superficial comments for now.  Are you planning on supporting word
variants?

Callish mach-ops, as generated from primops, are not sunk across,
but this behavior today is accidental (see Note [Foreign calls clobber
heap]), so be sure to adjust the notes.

Edward

Excerpts from Johan Tibell's message of 2014-05-04 03:10:12 -0700:
> Hi,
> 
> I found myself needing atomic operations on basic, mutable numeric values.
> I wrote a short design doc that I'd like feedback on:
> 
> https://ghc.haskell.org/trac/ghc/wiki/AtomicPrimops
> 
> I will try to implement these for 7.10.
> 
> -- Johan