Add an explicit end of call stack for x86_64

ostd/src/arch/x86/task/switch.S

@@ -31,6 +31,8 @@ first_context_switch: # (nxt: *const TaskContext)
   mov r13, [rdi + 32]
   mov r14, [rdi + 40]
   mov r15, [rdi + 48]
-  mov rax, [rdi + 56] # restore return address
-  mov [rsp], rax # for stack balance, must use mov instead of push
+  mov rax, [rdi + 56]
+  # for stack balance, we have to use mov instead of push here
+  mov qword ptr [rsp + 8], 0 # kernel_task_entry's return address
+  mov [rsp], rax             # first_context_switch's return address
   ret