sh: Add missing FORCE prerequisites in Makefile

make:

    arch/sh/boot/Makefile:87: FORCE prerequisite is missing

Add the missing FORCE prerequisites for all build targets identified by
"make help".

Fixes: e1f86d7b

 ("kbuild: warn if FORCE is missing for if_changed(_dep,_rule) and filechk")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>