[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();

src/kernel/debug.s

@@ -14,6 +14,13 @@ get_caller:
 	mov rax, [rbp+8]
 	ret
 
+global get_grandcaller
+get_grandcaller:
+	; No prelude - don't touch rsp or rbp
+	mov rax, [rbp]
+	mov rax, [rax+8]
+	ret
+
 global get_gsbase
 get_gsbase:
 	rdgsbase rax