Call the rust_main function from assembly

src/arch/x86_64/long_mode_init.asm

@@ -1,4 +1,5 @@
 global long_mode_start
+extern rust_main
 
 section .text
 bits 64
@@ -11,6 +12,10 @@ long_mode_start:
     mov fs, ax
     mov gs, ax
 
+    ; call the rust main
+    extern rust_main
+    call rust_main
+
     ; print `OKAY` to screen
     mov rax, 0x2f592f412f4b2f4f
     mov qword [0xb8000], rax