[Ada] Bootstrapping mainline GNAT fails

[Richard Kenner]

   --  If we get an exception while trying to do this test, then some error
   --  has occurred, and we simply say that the value is not known after all

   exception
      when others =>
         return False;
   end Compile_Time_Known_Value;

    It's not clear to me why this code would ever take an exception, though.

Nor me, but it does, though.  We ran into this on the ia64 port the other
day with a gnat1 that didn't correctly handle builtin setjmp/longjmp.