The atomic_compare_exchange_$n builtins have an exciting calling convention 
where the 4th ('weak') parm is dropped in a real call.  This caused 
gcc.dg/atomic-noinline.c to fail on PTX as the prototype didn't match the use.

Fixed thusly when emitting the ptx prototyp.  Also fixed  is  the subsequently 
uncovered type mismatch in the testcase -- atomic_is_lock_free's first parm is 
size_t not int.  Usually this mismatch is harmless even when one is 64 bits and 
the other 32. But again, PTX requires a match.

Applied to trunk.

nathan