Implement crt_sb_constructors(), which iterates over constructors in a
sandboxed binary executing them.

Provide a few more definitions on the path to also supporting
destructors.
Teach sandbox invocation code to recognise method '-1' as a request to
run object constructors.

This isn't really a hugely desirable approach: we'd rather add a special
instance entry point for the runtime to use, rather than using the
general invocation method -- this would avoid potential method-number
namespace collisions, as well as prevent other parties receiving object
references from invoking constructors additional times during execution.
However, this approach works for now, allowing static global capability
initialisations to work.
When libcheri initialises a new object instance, invoke the object's
constructor method so that global capabilities are properly initialised.