1.0.41.17: runtime: Make foreign_function_call_active work with threaded targets.

1.0.41.17: runtime: Make foreign_function_call_active work with threaded targets.

  * Add a slot to the thread structure for the active flag.

  * Make the existing global variable only show up on
unithread targets.

  * Introduce a wrapper macro to portably access the right
slot on both theaded and unithread targets.

  * KLUDGE things up to maintain the old behavior on x86oids
until someone gets around to fixing x86{,-64}-assem.S to set
foreign_function_call_active properly.