[Ada] Lock-free implementation of protected objects

[Arnaud Charlet]

> Having a default can't work, as s-atopri.adb needs access to the flag.
> Only the front end itself can use a default, not the run time.

Well, this means the current design is broken and s-atopri.adb needs to
be modified.

As we discussed, we cannot assume that System.Support_Atomic_Primitives
exists, since there will always be some target system*.ads files around
that don't have this flag (as shown by this discussion).

For a run-time (as opposed to compiler) parameter, s-parame is probably a
better place to start from, possibly using a separate.

Arno