Add related configuration and make files for tilegx.
The target can now build, though not run anything.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <BLU436-SMTP1588E5A03AD5E94B07E988B9660@phx.gbl>
Signed-off-by: Richard Henderson <rth@twiddle.net>