s390/string: Add -ffreestanding compile option to string.o

Use -ffreestanding for string.o to avoid that the compiler generates
calls into themselves for standard library functions like memset().

Reviewed-by: Juergen Christ <jchrist@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

arch/s390/boot/Makefile

@@ -25,6 +25,11 @@ KBUILD_CFLAGS += $(call cc-option, -Wno-default-const-init-unsafe)
 
 CFLAGS_sclp_early_core.o += -I$(srctree)/drivers/s390/char
 
+# string.o implements standard library functions like memset/memcpy etc.
+# Use -ffreestanding to ensure that the compiler does not try to "optimize"
+# them into calls to themselves.
+CFLAGS_string.o = -ffreestanding
+
 obj-y	:= head.o als.o startup.o physmem_info.o ipl_parm.o ipl_report.o vmem.o
 obj-y	+= string.o ebcdic.o sclp_early_core.o mem.o ipl_vmparm.o cmdline.o
 obj-y	+= version.o pgm_check.o ctype.o ipl_data.o relocs.o alternative.o

arch/s390/lib/Makefile

@@ -3,6 +3,11 @@
 # Makefile for s390-specific library files..
 #
 
+# string.o implements standard library functions like memset/memcpy etc.
+# Use -ffreestanding to ensure that the compiler does not try to "optimize"
+# them into calls to themselves.
+CFLAGS_string.o = -ffreestanding
+
 lib-y += delay.o string.o uaccess.o find.o spinlock.o tishift.o
 lib-y += csum-partial.o
 obj-y += mem.o