Ensure that the usnic BTL is only built on 64 bit Linux platforms.

Reviewed by Dave Goodell.

cmr=v1.7.4:reviewer=ompi-rm1.7:subject=Ensure the usnic BTL only builds on 64 bit Linux
One last corner case - when encountering an overload condition (e.g., by comm_spawning more procs than we have cores) and we are using the default binding policy, do *not* bind the new procs to anything as this can cause major problems. Instead, let the spawn succeed since the user didn't specifically ask to be bound, and leave the new procs as unbound.

Refs #4077