OSHMEM: spml ikrit: add hardware rdma endpoint

Commits add mxm endpoint that can be used for transports with hw rdma capabilites. Atomic operations are set to use this endpoint. 

It is now possible to run UD transport to get a good scalability and RC to benefit from IB hardware atomics.

Commits from: open-mpi/ompi@1bcc88cfb1a0868e733802d343ddf16ffd2b93a0 
to: open-mpi/ompi@643e64497d531830111ed7a1ddf5bebceea9d4a3

@miked-mellanox @jladd-mlnx
 
Minor style nit: please put {} around all blocks, even 1-line blocks.