scheduler: have a separate stack for yield

kernel-src/include/x86-64/arch/context.h

@@ -60,7 +60,7 @@ typedef struct {
 }
 
 void arch_context_switch(context_t *context);
-void arch_context_saveandcall(void (*fn)(context_t *context));
+void arch_context_saveandcall(void (*fn)(context_t *context), void *stack);
 
 #include <string.h>
 

kernel-src/include/x86-64/arch/cpu.h

@@ -24,6 +24,7 @@ typedef struct cpu_t {
 	thread_t *thread;
 	thread_t *idlethread;
 	timerentry_t schedtimerentry;
+	void *schedulerstack;
 } cpu_t;
 
 #define CPU_HALT() asm volatile("hlt")