Factorize elf32 / elf64 code in Multiboot loader. This will
prevent it from getting out of sync again.
* loader/i386/multiboot.c (grub_multiboot_is_elf32,
grub_multiboot_load_elf32, grub_multiboot_is_elf64,
grub_multiboot_load_elf64): Move from here ...
* loader/i386/multiboot_elfxx.c (grub_multiboot_is_elf,
grub_multiboot_load_elf): ... to here (new file).
