s390/kasan: enable stack and global variables access checks

By defining KASAN_SHADOW_OFFSET in Kconfig stack and global variables
memory access check instrumentation is enabled. gcc version 4.9.2 or
newer is also required.

Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>