s390/kasan: replace some memory functions

Follow the common kasan approach:

    "KASan replaces memory functions with manually instrumented
    variants.  Original functions declared as weak symbols so strong
    definitions in mm/kasan/kasan.c could replace them. Original
    functions have aliases with '__' prefix in name, so we could call
    non-instrumented variant if needed."

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