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 <[e-mail]>
Acked-by: Heiko Carstens <[e-mail]>

arch/s390/boot/Makefile

···
 	sh -x  $(srctree)/$(obj)/install.sh $(KERNELRELEASE) $(obj)/bzImage \
 	      System.map "$(INSTALL_PATH)"
 
-chkbss := $(OBJECTS)
-chkbss-target := $(obj)/startup.a
+chkbss := $(obj-y)
+chkbss-target := startup.a
 include $(srctree)/arch/s390/scripts/Makefile.chkbss

arch/s390/boot/compressed/Makefile

···
 $(obj)/piggy.o: $(obj)/vmlinux.bin$(suffix-y) FORCE
 	$(call if_changed,objcopy)
 
-chkbss := $(filter-out $(obj)/piggy.o $(obj)/info.o,$(OBJECTS))
-chkbss-target := $(obj)/vmlinux.bin
+chkbss := $(filter-out piggy.o info.o, $(obj-y))
+chkbss-target := vmlinux.bin
 include $(srctree)/arch/s390/scripts/Makefile.chkbss

arch/s390/scripts/Makefile.chkbss

···
 # SPDX-License-Identifier: GPL-2.0
 
+chkbss-target ?= built-in.a
+$(obj)/$(chkbss-target): chkbss
+
+chkbss-files := $(addsuffix .chkbss, $(chkbss))
+clean-files += $(chkbss-files)
+
+PHONY += chkbss
+chkbss: $(addprefix $(obj)/, $(chkbss-files))
+
 quiet_cmd_chkbss = CHKBSS  $<
-define cmd_chkbss
-	rm -f $@; \
+      cmd_chkbss = \
 	if ! $(OBJDUMP) -j .bss -w -h $< | awk 'END { if ($$3) exit 1 }'; then \
 		echo "error: $< .bss section is not empty" >&2; exit 1; \
 	fi; \
 	touch $@;
-endef
 
-chkbss-target ?= $(obj)/built-in.a
-ifneq (,$(findstring /,$(chkbss)))
-chkbss-files := $(patsubst %, %.chkbss, $(chkbss))
-else
-chkbss-files := $(patsubst %, $(obj)/%.chkbss, $(chkbss))
-endif
-
-$(chkbss-target): $(chkbss-files)
-targets += $(notdir $(chkbss-files))
-
-%.o.chkbss: %.o
+$(obj)/%.o.chkbss: $(obj)/%.o
 	$(call cmd,chkbss)