[kernel] Add get_grandcaller debug function

This should probably be replaced with a function taking the number of
steps back, but this is quick and useful.

src/kernel/debug.h

@@ -10,6 +10,7 @@ extern "C" {
 	uintptr_t get_rsp();
 	uintptr_t get_rip();
 	uintptr_t get_caller();
+	uintptr_t get_grandcaller();
 	uintptr_t get_frame(int frame);
 	uintptr_t get_gsbase();
 	void _halt();