Fix usage of hpx_generic_coroutine_context and get tests passing on powerpc

After recent cmake changes some cmake vars and #defines were inconsistently used - leading to use of generic coroutine contexts even when disabled. Fix this and also provide correct settings for BlueGene IO node compilation on powerpc architectures and support __powerpc__ flag in relevant places.
What is this for? Did you forgot to remove that or is there something you're planning to add, still?