Add missing atomic_domain operations

[Dan Bonachea]

GASNet-EX remote atomics support includes the following operations, all of which are currently missing at the UPC++ level:

1. atomic_domain.(fetch_)mult
2. atomic_domain.(fetch_)min
3. atomic_domain.(fetch_)max
4. atomic_domain.(fetch_)bit_and - only for integer types
5. atomic_domain.(fetch_)bit_or - only for integer types
6. atomic_domain.(fetch_)bit_xor - only for integer types

We should add them to the UPC++ spec and implementation.

Now that the atomics spec is factored, the spec work should be a simple matter of adding them to the operation enum and function table. The implementation should be similarly mechanical.

@sbbaden has an immediate use case for atomic_domain.bit_xor(), which is required for implementing HPC Challenge RandomAccess (GUPS) using remote atomics - which is likely to be best fine-grained implementation we can produce for large-scale runs on Cray XT and other systems with offload support for atomics.

[Dan Bonachea]

Resolved in 9a196b8 (pull request akamil/#1)