s390/kasan: improve string/memory functions checks

Avoid using arch specific implementations of string/memory functions
with KASAN since gcc cannot instrument asm code memory accesses and
many bugs could be missed.

Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/boot/string.c

@@ -2,6 +2,7 @@
 #include <linux/ctype.h>
 #include <linux/kernel.h>
 #include <linux/errno.h>
+#undef CONFIG_KASAN
 #include "../lib/string.c"
 
 int strncmp(const char *cs, const char *ct, size_t count)