arch: syscalls: add missing FORCE and fix 'targets' to make if_changed work

arch: syscalls: add missing FORCE and fix 'targets' to make if_changed work

The rules in these Makefiles cannot detect the command line change
because the prerequisite 'FORCE' is missing.

Adding 'FORCE' will result in the headers being rebuilt every time
because the 'targets' additions are also wrong; the file paths in
'targets' must be relative to the current Makefile.

Fix all of them so the if_changed rules work correctly.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>