AFL stability fix: avoid inlining of lazy forcing

[ncik-roberts]

Extend the fix originally made in #1754 to be more airtight.

That PR has more context on why it's important for the forcing of lazy values to not be inlined when AFL instrumentation is enabled.

The present PR makes it obviously true that this comment (from just above the change) holds:

    (* Disable inlining optimisation if AFL instrumentation active,
       so that the GC forwarding optimisation is not visible in the
       instrumentation output.
       (see https://github.com/stedolan/crowbar/issues/14) *)

[gasche]

Obviously correct.