[gdbserver/LynxOS]: Incomplete thread list after --attach

The current implementation is forgetting to populate the thread list
when attaching to the process. This results in an incomplete list of
threads when debugging a threaded program.

Unfortunately, as the added comments hints, there appears to be
no way of getting the list of threads via ptrace, other than by
spawning the "ps" command, and parsing its output. Not great,
but it appears to be the best we can do.

gdb/gdbserver/ChangeLog:

        * lynx-low.c (lynx_add_threads_after_attach): New function.
        (lynx_attach): Remove call to add_thread.  Add call to
        lynx_add_threads_after_attach instead.