s390: make chkbss work with clang

llvm skips an empty .bss section entirely, which makes
the check fail with an unexpected error:

/tmp/binutils-multi-test/bin/s390x-linux-gnu-objdump: section '.bss' mentioned in a -j option, but not found in any input file
error: arch/s390/boot/compressed/decompressor.o .bss section is not empty
../arch/s390/scripts/Makefile.chkbss:20: recipe for target 'arch/s390/boot/compressed/decompressor.o.chkbss' failed

Change the check so we first see if a .bss section exists
before trying to read its size.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/scripts/Makefile.chkbss

@@ -11,7 +11,8 @@ chkbss: $(addprefix $(obj)/, $(chkbss-files))
 
 quiet_cmd_chkbss = CHKBSS  $<
       cmd_chkbss = \
-	if ! $(OBJDUMP) -j .bss -w -h $< | awk 'END { if ($$3) exit 1 }'; then \
+	if $(OBJDUMP) -h $< | grep -q "\.bss" && \
+	   ! $(OBJDUMP) -j .bss -w -h $< | awk 'END { if ($$3) exit 1 }'; then \
 		echo "error: $< .bss section is not empty" >&2; exit 1; \
 	fi; \
 	touch $@;