Minor conditionals to enable building Scheduler Simulator on GNU/Linux

- rtems/score/assert.h: Scheduler Simulator uses glibc assert.h on GNU/Linux.
  This will likely need to be adjusted more for other host compilers and
  C libraries. Also disable _Assert_Not_reached() because some of these
  paths do actually return to the called on the Scheduler Simulator.

- basedefs.h: Do not use noreturn attribute when on Scheduler Simulator.
  Paths which context switch can return to the command interpreter on
  the Scheduler Simulator.
cpusetimpl.h: Add _CPU_set_Set() and improve Doxygen