kernel: Add missing smp memory barrier calls. Set max cpu to 1

* Set max cpu to 1 for PPC until atomic functions are finished
* We have atomic functions inline in the kernel and assembly
  code in libroot post-scheduler merge... isn't that a lot of
  duplication?
kernel: Rework ppc (and m68k) atomic functions post-scheduler

* Make atomic function more like current x86
* Remove fake fallback atomic code for ppc as hardware spinlocks exist