Load null selectors to all data registers

This is required for some instructions such as iretq (ss must be valid or 0).

(cherry picked from commit 8f8b46a9b6)

src/arch/x86_64/long_mode_init.asm

@@ -13,6 +13,14 @@ extern rust_main
 section .text
 bits 64
 long_mode_start:
+    ; load 0 into all data segment registers
+    mov ax, 0
+    mov ss, ax
+    mov ds, ax
+    mov es, ax
+    mov fs, ax
+    mov gs, ax
+
     ; call rust main (with multiboot pointer in rdi)
     call rust_main
 .os_returned: