Pass multiboot pointer as argument to rust_main

src/arch/x86_64/boot.asm

@@ -19,6 +19,9 @@ section .text
 bits 32
 start:
     mov esp, stack_top
+    ; Move Multiboot info pointer to edi to pass it to the kernel. We must not
+    ; modify the `edi` register until the kernel it called.
+    mov edi, ebx
 
     call test_multiboot
     call test_cpuid