[cpp-threads] atomic operations package

[Peter Dimov]

Boehm, Hans wrote:

> In my view, the atomic<> template actually buys us something,
> in that it can be applied to enumeration types, etc.

There is no reason for a function-style atomic intrinsic to not work on 
arbitrary POD types, as long as their sizes and alignments satisfy the 
atomicity constraints.

We may (or may not) want to provide an atomic<T> wrapper that enforces the 
appropriate size or alignment for some Ts (enums, chars), but this is a 
separate "convenience" addition and not a necessity. I'd omit it.