[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.
