s390: make built-in.a not directly depend on *.o.chkbss files

When I was refactoring cmd_ar_builtin in scripts/Makefile.build,
I noticed the build breakage of s390.

Some Makefiles of s390 add extra dependencies to built-in.a;
built-in.a depends on timestamp files *.o.chkbss, but $(AR) does
not want to include them into built-in.a.

Insert a phony target 'chkbss' in between so that $(AR) can take
$(filter-out $(PHONY), $^) as input.

While I was here, I refactored Makefile.chkbss a little bit.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>