lang/gnat12: adding GNAT from Gcc-12

This Ada compiler is built from Gcc-12, with help from Stephane Carrez
(Ada France).

The first time it uses lang/gcc6-aux as a bootstrap, but it can produce
a bootstrap for the next time, either for this version or for the next
ones.

Yet to be done:
- plug it into Mk/Uses/ada.mk
- build more bootstrapping assets
  (ATM they exist only for amd64 on FreeBSD 13 and 14)
- add a test target
- try to cross-compile for more platforms?