Java: add flag_use_atomic_builtins

[Boehm, Hans]

[Partially replying to myself]
> From:  Boehm, Hans
> 
> At the risk of asking a stupid question, shouldn't all the 
> code inside gcc gradually migrate towards using the C++0x 
> (and probably C1x) atomics, which seem to be generally 
> supported by gcc 4.4?
> 
> There are known issues with __sync (no atomic loads and 
> stores, underspecified ordering), which is why there wasn't 
> much of an effort topush the __sync interface into C++0x.
> 
> Hans
> 
OK.  That was largely a stupid question, since we're talking about the compiler implementation of those primitives, which presumably are shared with the atomic<T> implementation?

Hans