Use __atomic builtins, remove volatile fields

[DaoWen]

With proper use of __atomic builtins, we should be able to introduce the needed memory barriers, and remove the volatile qualifier from a few places in our code. It's not clear that volatile is actually doing anything useful right now anyway (our usage of the legacy __sync builtins might be sufficient).

[sbak5]

volatile is just a keyword to prevent compiler optimizations on the pointers because the pointers are being updated across threads. This is orthogonal to memory consistency.

memory barrier and atomic operations should be called in orthogonal to the volatile keywords.