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.
Shouldn't this be `HPX_HAVE_GENERIC_COROUTINE_CONTEXT` now?