LoongArch: Add dependency between vmlinuz.efi and vmlinux.efi

A common issue in Makefile is a race in parallel building.

You need to be careful to prevent multiple threads from writing to the
same file simultaneously.

Commit 3939f334 ("ARM: 8418/1: add boot image dependencies to not
generate invalid images") addressed such a bad scenario.

A similar symptom occurs with the following command:

  $ make -j$(nproc) ARCH=loongarch vmlinux.efi vmlinuz.efi
    [ snip ]
    SORTTAB vmlinux
    OBJCOPY arch/loongarch/boot/vmlinux.efi
    OBJCOPY arch/loongarch/boot/vmlinux.efi
    PAD     arch/loongarch/boot/vmlinux.bin
    GZIP    arch/loongarch/boot/vmlinuz
    OBJCOPY arch/loongarch/boot/vmlinuz.o
    LD      arch/loongarch/boot/vmlinuz.efi.elf
    OBJCOPY arch/loongarch/boot/vmlinuz.efi

The log "OBJCOPY arch/loongarch/boot/vmlinux.efi" is displayed twice.

It indicates that two threads simultaneously enter arch/loongarch/boot/
and write to arch/loongarch/boot/vmlinux.efi.

It occasionally leads to a build failure:

  $ make -j$(nproc) ARCH=loongarch vmlinux.efi vmlinuz.efi
    [ snip ]
    SORTTAB vmlinux
    OBJCOPY arch/loongarch/boot/vmlinux.efi
    PAD     arch/loongarch/boot/vmlinux.bin
  truncate: Invalid number: ‘arch/loongarch/boot/vmlinux.bin’
  make[2]: *** [drivers/firmware/efi/libstub/Makefile.zboot:13:
  arch/loongarch/boot/vmlinux.bin] Error 1
  make[2]: *** Deleting file 'arch/loongarch/boot/vmlinux.bin'
  make[1]: *** [arch/loongarch/Makefile:146: vmlinuz.efi] Error 2
  make[1]: *** Waiting for unfinished jobs....
  make: *** [Makefile:234: __sub-make] Error 2

vmlinuz.efi depends on vmlinux.efi, but such a dependency is not
specified in arch/loongarch/Makefile.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>