Make the ELF entry point a physical address

This way, the bootloader doesn't have to translate the entry point.
This also makes xv6 multiboot-compliant and follows the convention
used by Linux.

bootmain.c

@@ -43,7 +43,7 @@ bootmain(void)
 
   // Call the entry point from the ELF header.
   // Does not return!
-  entry = (void(*)(void))(elf->entry - KERNBASE);
+  entry = (void(*)(void))(elf->entry);
   entry();
 }