s390/boot: fix zstd build for -march=z900

zstd decompression uses __builtin_clz() which fails back to __clzdi2()
when the kernel is built for older hardware like z900. This leads to
build failures like the following:
s390x-11.1.0-ld: /devel/src/kernel/arch/s390/boot/compressed/../../../../lib/zstd/bitstream.h:148: undefined reference to `__clzdi2'

Fix that by optionally including lib/clz_ctz.c into the decompressor.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 7b034d9 ("s390/boot: add zstd support")
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Link: https://lore.kernel.org/r/patch-1.thread-f0f589.git-f0f58936888f.your-ad-here.call-01627564869-ext-2765@work.hours
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/boot/compressed/Makefile

@@ -11,6 +11,7 @@ UBSAN_SANITIZE := n
 KASAN_SANITIZE := n
 
 obj-y	:= $(if $(CONFIG_KERNEL_UNCOMPRESSED),,decompressor.o) info.o
+obj-$(CONFIG_KERNEL_ZSTD) += clz_ctz.o
 obj-all := $(obj-y) piggy.o syms.o
 targets	:= vmlinux.lds vmlinux vmlinux.bin vmlinux.bin.gz vmlinux.bin.bz2
 targets += vmlinux.bin.xz vmlinux.bin.lzma vmlinux.bin.lzo vmlinux.bin.lz4

arch/s390/boot/compressed/clz_ctz.c

@@ -0,0 +1,2 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "../../../../lib/clz_ctz.c"