[Ada] Bootstrapping mainline GNAT fails

[Robert Dewar]

<<If exceptions are used only to signal fatal errors, then you could
avoid ever actually _throwing_ an exception; instead, the runtime
routines that normally cause exceptions to be thrown, would be swapped
out for custom versions that called fatal_error()... sort of like the
abort() macro in the other language front ends.
>>

There are actually almost no fatal errors in GNAT that terminate
compilation. The only common case is where the compiler blows up
somehow after already detecting an error.

Even then we try to avoid such blow ups terminating compilation (that's
what the exception handler in this case is all about).