This macro is used to implement global serialisation, typically by
disabling IRQs. On the unix port, if threading is enabled, use the
existing thread mutex (that protects the thread list structure) for this
purpose. Other places in the code (eg the scheduler) assume this macro
will provide serialisation.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.
