Per-cpu variables can be used from contexts where preemption is disabled
(such as interrupts) or when migration is impossible (pinned threads) for
managing data that is replicated for each cpu.

The API is a smart pointer to a variable which resolves to the object's
location on the current cpu.

Define:

   #include <osv/percpu.hh>

   PERCPU(int, my_counter);
   PERCPU(foo, my_foo);

Use:

   ++*my_counter;
   my_foo->member = 7;