sched: thread: add cpu context pointer usable during interrupts and syscalls

include/kernel/thread.h

@@ -8,6 +8,8 @@
 
 #define THREAD_KSTACK_ORDER VM_PAGE_4K
 
+struct ml_cpu_context;
+
 enum thread_state {
 	THREAD_READY = 1,
 	THREAD_SLEEPING = 2,
@@ -39,6 +41,9 @@ struct thread {
 	virt_addr_t tr_ip, tr_sp, tr_bp;
 	virt_addr_t tr_cpu_user_sp, tr_cpu_kernel_sp;
 
+	/* only valid within an interrupt or syscall context */
+	struct ml_cpu_context *tr_irqctx;
+
 	struct ml_thread tr_ml;
 	struct runqueue *tr_rq;