sh: boot: avoid unneeded rebuilds under arch/sh/boot/compressed/

Even if none of source code is updated, the following are every time
rebuilt:

  CC      arch/sh/boot/compressed/cache.o
  SHIPPED arch/sh/boot/compressed/ashiftrt.S
  AS      arch/sh/boot/compressed/ashiftrt.o
  SHIPPED arch/sh/boot/compressed/ashldi3.c
  CC      arch/sh/boot/compressed/ashldi3.o
  SHIPPED arch/sh/boot/compressed/ashrsi3.S
  AS      arch/sh/boot/compressed/ashrsi3.o
  SHIPPED arch/sh/boot/compressed/ashlsi3.S
  AS      arch/sh/boot/compressed/ashlsi3.o
  SHIPPED arch/sh/boot/compressed/lshrsi3.S
  AS      arch/sh/boot/compressed/lshrsi3.o
  LD      arch/sh/boot/compressed/vmlinux
  OBJCOPY arch/sh/boot/zImage

Add build artifacts to 'targets' as needed.

I turned the library files to check-in files. It is simpler than
copying from arch/sh/lib/ at build-time.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: Rich Felker <dalias@libc.org>