[mm] Allow to disable the mm based `malloc` at runtime

The goal is to enable the mm based `malloc` only when needed and fall
back to the (more efficient) builtin/next implementation when it is
not needed:

  * run instrospection-less jobs without it;

  * whene the MC and the application are in different processes, the
    MC will be able to run with the standard `malloc` and the
    application will use mm.

As malloc is needed very early in the application initialisation, an
environment variable is used to change the behaviour.