fix bug in the initialization of the stack pointer

arch/x86/kernel/entry64.asm

@@ -48,14 +48,6 @@ mboot:
     dd MULTIBOOT_HEADER_FLAGS
     dd MULTIBOOT_CHECKSUM
     
-    ; AOUT kludge - must be physical addresses. Make a note of these:
-    ; The linker script fills in the data for these ones!
-    ; dd mboot
-    ; dd code
-    ; dd bss
-    ; dd end
-    ; dd start
-
 ALIGN 4
 ; we need already a valid GDT to switch in the 64bit modus
 GDT64:                           ; Global Descriptor Table (64-bit).
@@ -253,7 +245,7 @@ start64:
     mov ss, ax
 ; set default stack pointer
     extern default_stack_pointer
-    mov rsp, default_stack_pointer
+    mov rsp, [default_stack_pointer]
 ; interpret multiboot information
     extern multiboot_init
     mov rdi, rbx