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

src/arch/x86_64/long_mode_init.asm

@@ -20,7 +20,7 @@ bits 64
 long_mode_start:
     call setup_SSE
 
-    ; call rust main
+    ; call rust main (with multiboot pointer in rdi)
     call rust_main
 .os_returned:
     ; rust main returned, print `OS returned!`