Make bootsector stub 16-bit-only (x86_64)

Hi!

The x86 bzImage contains a stub to inform people that it is not possible any more to run a Linux kernel by catting it to a floppy and then booting from it. This was meant to be all 16-bit code. The first instruction, however, ended up as being coded as a 16:32-bit far jump. I assume the intention was a 16:16-bit far jump.

This patch changes only x86_64.

Greetings,
Alexander

---

index 011b7a4..ae9df0d 100644
@@ -44,7 +44,7 @@ #endif
 _start:
 
 	# Normalize the start address
-	jmpl	$BOOTSEG, $start2
+	jmpw	$BOOTSEG, $start2
 
 start2:
 	movw	%cs, %ax