GHC branch "atomics" pushed

Hi all, There should be very little that is controversial about this branch because it adds primops upon which nothing else depends and which users will not see directly. But for now I've added it to a branch called "atomics" (likewise use the "atomics" testsuite branch of course). As soon as Carter and I work out a bit more of the future roadmap I'll merge it into master. Carter, would you like to use this branch to develop the improved LLVM support for the primops that are now added there? Cheers, -Ryan P.S. Carter, as you requested I'll add an extra trac ticket for discussion.