[tex-live] oversimplyfied test for lzmadec existence

[Karl Berry]

      TLPDB::setting: no available_architectures, returning empty list ...

    to stderr while running.  What about turning such warning messages into
    debug messages (see attached patch)?

That change affects more than just available_architectures, though.
That doesn't seem like a good idea.  It's also a drag that it occurs in
two places.

We could make the debugification be conditional, of course, but I wonder
if there's a cleaner way to make it work.  Maybe somehow "unknown" could
get written into available_architectures, for instance?  Something else?

I'm just blindly speculating, I've never tried to run it on an unknown
architecture.