runtime,sync/atomic: unify API for runtime/internal/atomic and sync/atomic

[dvyukov]

We need to unify interface of atomic operations between runtime package and sync/atomic
package. And then teach gc to replace functions in both packages with intrinsics where
possible.

This is followup to https://golang.org/issue/4947 which was about
atomic intrinsics for runtime C code.

[rsc]

There's no evidence in profiles that this is important for Go 1.5.