Clean up atomic primitives in libcore

[bblum]

nasty organisation in pipes and in arc.

[eholk]

I think ARCs call into the runtime to do atomic operations. Now that we have atomic intrinsics, we can use those instead.

I suspect some of the nasty organization in pipes might have been working around compiler bugs, like #3005. The workarounds may not actually be effective though, it could have just been stuff I tried when looking for a workaround that I never removed.

[graydon]

I think this is closable now (see #5042, #6732, #6740). We have a good atomics library now, arcs use them and/or the atomic intrinsics, and pipes use the atomic intrinsics. Reopen if you disagree.