Update xcompiler Dockerfile from Trusty to Xenial 

So the crosstool-ng Dockerfile is way back on Trusty, and can now be jumped all the way up to the next LTS.

I think that will implicitly give us a newer GCC release too, on the assumption that the GCC is just coming from the host OS.

Trusty comes with GCC 4.8.2 (October 16, 2013).   Xenial comes with GCC 5.3.1 (December 2015)
