* gdb.threads: New directory with some crude multi-threaded

    * gdb.threads: New directory with some crude multi-threaded
        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.