gdb tests (step.exp and step2.exp).
* config/mt-lynx (STEP_EXECUTABLE): Define.
(THREADFLAGS): Define.
Hiro's test for 6100 and a crude test of my own. They both need some
major cleanup for non-lynx hosts; both are also a little too loose in
their checks. But I'm too burnt out on lynx to address either concern.
