Gnat in GCC Again

[dewar@gnat.com]

We are currently discussing whether to do this before or after the 3.0
release, but in any case it will definitely happen in the very near future.
We are just in the process of releasing GNAT 3.14, so this is a good point
at which to integrate the sources.

We have not quite got 100% correct passage of all tests, but we are
very close (within a few tests), and near enough that that is not a
factor at this stage.