[kernel] Add get_caller function

Added a function to get the callsite of the current function invocation.

src/kernel/debug.h

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

src/kernel/debug.s

@@ -8,6 +8,12 @@ get_rip:
 	pop rax ; do the same thing as 'ret', except with 'jmp'
 	jmp rax ; with the return address still in rax
 
+global get_caller
+get_caller:
+	; No prelude - don't touch rsp or rbp
+	mov rax, [rbp+8]
+	ret
+
 global get_gsbase
 get_gsbase:
 	rdgsbase rax