Move the prototype for savectx from cpu.h to pcb.h, as it is on other

platforms, as well as putting it in an #ifdef KERNEL block.

MFC after:	2 weeks

sys/powerpc/include/cpu.h

@@ -98,7 +98,4 @@ void	cpu_reset(void);
 void	fork_trampoline(void);
 void	swi_vm(void *);
 
-/* XXX the following should not be here. */
-void	savectx(struct pcb *) __returns_twice;
-
 #endif	/* _MACHINE_CPU_H_ */

sys/powerpc/include/pcb.h

@@ -89,6 +89,7 @@ extern struct pmap *curpm;
 extern struct proc *fpuproc;
 
 void	makectx(struct trapframe *, struct pcb *);
+void	savectx(struct pcb *) __returns_twice;
 
 #endif
 #endif	/* _MACHINE_PCB_H_ */